How to discover devices that still use old AD servers for DNS?

Just looking to make things easier for myself.  I am working on phasing out our old Microsoft AD domain controllers, which were previously used to internal DNS resolution, with new ones.  I've updated our DHCP to the new domain controllers, now I'm trying to weed out all the devices that have statically assigned DNS settings to manually update them.

Now that all DHCP clients should be getting DNS from the new servers, I'm wondering if there's a way (powershell?) to poll DNS activity on the old servers to generate myself a list of IP addresses that are still getting DNS from the old servers?  I'm thinking this has probably come up before, but so far I haven't found anything, wondering if someone knows the answer already and can point me at it?
TatankasaAsked:
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nociSoftware EngineerCommented:
Use wireshark on the old DNS server and monitor for incoming UDP port 35 traffic.
If there is then the source  address of that packet will show WHO is (still) querying your old DNS server.
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DrDave242Senior Support EngineerCommented:
Noci is correct; Wireshark will do the trick, and I can't think of a better method off the top of my head. I believe he meant port 53 instead of 35, though. :)
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TatankasaAuthor Commented:
Sounds good guys, thanks for the tip!
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