Software forensics

I'm an experienced digital forensic expert and can certainly do all the elements of finding what the two bodies of code have in common, including near duplication of sections, etc.

This is VERY general, but other things to look for here include his access to your code, how and when he might have misappropriated it, etc. Did he copy it from a computer? Transfer it to another computer via flash drive or email? Questions like that.

What I cannot do is opine on code functionality and such. If that's needed, I would reach out to contacts with that expertise.

What part of world/country does this apply to?
What does the end product do? Is he using it for the same purpose you intended it to or was it repurposed.
You say that he ha it all rewritten ... Does that mean that he admits to using your code as a template for the rewrite?

Forget it.  

Unless you can get to more authoritative evidence than what you now have it will be an uphill battle.  You will spend more money on lawyers and forensic experts than you could make all year writing VB.  

We do forensic work and have experience with software cases up to federal court.  The principal issues are NOT details of your code but ARE business agreements and direct access to the other party's computers.  If they are not clearly on your side you will lose (money, time, health).

Whether or not you have a legal case, the bitterness that you have will eat you up.  That's a win for the other side.

Just a little more background.  This was a client I worked for over a period of 3 years.  We are both in the US.  He is in KY and I am in WI. His bill continued to grow faster than his payments.  Multiple attempts to get him to agree to a payment plan just didn't work out.  I finally requested he sign a document stating he would pay $2000 a month.  He blew-up, got very angry that I was forcing him to sign something, but eventually he did.  I few days later, he locked me out of everything (the web site which reads a live data feed and turns the data into multiple graphs, charts and widgets on his subscription based financial site).  Did not hear from him again until I turned it over to a collection agency in KY.  He claims the code was so bad it had to be completely re-written, so he did not have to pay me.  Fact is the site ran flawlessly.  He was hoping I would not pursue this due to the time and money required, but I have. We won a request from the court for his code and the copy I have to be sent to a 3rd-party to see if he is still running my code or not.  If he is, then we hope that forcing him to turn his code over may change his mind about paying his bill.  I hope that answers the questions asked so far.

Having been on both sides of such problem I agree with gmit.
At the end only lawyers will win and unless you prevail you will have to pay for the expert fees. And even if you do, there will be an appeal and many other ways to avoid paying.
From an experience I can tell you that something like this will cost you at minimum $50k and it can go up from there fast.
Code in an art and not an exact science so judging performance, coding style and such is extremely difficult. Further things can be complicated by the developers choice of platform and tools used by you and what is used now.
You may not want to hear this and by any means I mean no insult by saying this... comparatively speaking you are not talking a lot of money. I wish that I listened to advice like I am giving... Use this as an  learning experience for not staying on top of your billing and allowing  to be taken advantage of by continuing work w/o pay...
I wish you best of luck.

I will echo some of what others have said. I spent a year or so inventing a product, getting it to market, etc., and within a week there were Chinese knockoffs on the shelf at half my price. I have great respect for intellectual property and the way it feels to have it stolen. I ultimately spent four years in federal court suing infringers. Even though I legally prevailed in every single instance, it was one of the most emotionally gruelling periods of my life. And despite a love for inventing, I stopped cold rather than subject myself to all that again.

All that said, at the end of the day it's your decision. I'm a DF expert, NOT a lawyer, so don't interpret any of this as legal advice. It sounds like your case would be multi-faceted, establishing the access he had to your code, how he might have misappropriated it, what he did with it afterward, and then how he has benefited from such a misappropriation. Lawyers are expensive, and we experts are expensive. You're looking at spending a ton of cash.

Thank you all for your honest bits of advice. I do have an advantage here. This was turned over to a collections agency. They promised they would pay any legal fees if they could not collect.  So far they have kept their word, and we have had pre-trial hearing in which the judge felt there was money to be paid, and encourage us to go to trial.  Aside from the collections agency's lawyer, a second lawyer has been donating his time because he was a client of mine and simply wants to help.  Everyone involved is feeling positive about winning if we go to court.  That would cost me some traveling expenses, and force me to be away from an ill family member I am caring for, but if need be I will have to find a way to get there.  Both lawyers felt that asking for a copy of the web site to prove the code was not re-written would but pressure on the defendant, and perhaps that would make him more likely to pay the bill.  If it was not for the free legal resources I have, I never would have pursued it.

So, back to the original question: where can I find someone who could compare the two web sites and their code to determine if he is still using my code?

What area of the country are you? Different states have different licensing requirements for forensic work like this.

CopOMatic
I believe he said KY an WI, I think that you are safe w/o licencing there. I would be curious to know if you think different.
Personally I have bad feeling about this situation and so I am going to stay away.
One last bit of advice and it is NOT legal advice, just my opinion ;-)
From my experience legal fees usually do not include fees associated with experts. Be very careful about that so you do not find yourself owing a bunch of money if the %$#@*&^ lawyers settle your case.
I was recently involved in a case and this exact think happened.

I am in Wisconsin.  The client and collection agency are in Kentucky.

Thx, insidetech. I missed that. I pinged one of my lists to see if anyone has specifics on requirements in WI and KY.

OP, where is the court action taking place?

Seeing a number of you have experience with situations like this before, and I have explained my intent is to pressure the client, I wonder if I should drop the code comparison, and just ask for prove he had it re-written.  Such things as invoices.  Your thoughts.

That's really getting into legal strategy for your lawyer, but the first thing I'd say from the other side in response to that approach is:  "No, I don't have any proof of rewriting. So what? You have no proof that I ever had anything of yours to rewrite."

ChopOMatic,  I have 3 years of invoices, hundreds of emails, and the clients own admission that I wrote his web site.

Understood. Just playing devil's advocate.

ChopOMatic, you make a good one.  Continue to do so when you feel like. :-)

Where is the legal process? Has a suit been filed? If so, what are the claims?

The collection agency took it to court.  It was a pre-trial (I think they called it that) hearing when a judge give his opinion if there is any merit to the case.  He, the judge, said there is certainly some money here that needs to be paid.  Both lawyers encourage me to continue because the judges comments clearly favored our side.  The collection agency lawyer want to go to court right away, but I suggested we try to get proof that the client was lying.  He claims my code was so bad that he HAD to have it re-written.  In truth, it ran perfectly, and his e-mails to me a fully of nothing but praise.  Both lawyers thought this was a good idea.  If we can show he is still using my code, and he knows we have the proof, then he can be served with a cease-and-desist order.  I am not planning to do that, but it will put additional pressure on the client, and he will not be able to claim he has a right not to pay the bill because I did such a sloppy job.  We have asked the client to produce the code for some time, but he (actually his drinking buddy aka his lawyer) kept refusing.  Last week we went to court to demand he cough it up, and the judge ordered him to do so.  It is now my responsibility to find a third-part to compare the two sets of code.  Of course, he has had 3 years to re-write the code, so he may produce different code.  But if we ask for invoices he paid, and who re-wrote what, I think if would show when the code was re-written.  I didn't tell the lawyer to ask for that, but I did mention it later after the court hearing.
So, we went to court.  The court told him to cough it up.  I have been told to find a 3rd-party to compare the code.

Is the court in Wisconsin or Kentucky?

Kentucky.

ChopOMatic,  are you still interested in this?

Sure. I moved this past week and have been miserably busy with that.

ChopOMatic, I think the lawyer wants the person to go to Kentucky.  I do not know why.  He never mentioned that before.  I waiting to hear back from him to see if a trip to Knetucky is really required.