PHP: How can I detect if a user came from a Search Engine

I have a site where I award user's Viz (currency) for giving users their referral url and when a person being referred clicks it, they person referring them gets the Viz.  However, a problem is that Google has indexed some of those url's with the referral code in them so what I want to do is detect if the referred user is coming from a Search engine and not award the Viz.