Question : DNS server not returning requests

We have setup a new non-AD integrated DNS server to serve requests of our internal websites. A few users are unable to browse internal testing sites but can browse to forward public facing production web sites that also have DNS records on the new server. I think that the users pc's are not requesting the data from the new primary DNS server but are finding it on our secondary AD DNS servers which would also know how to locate the production websites but not the internal testing sites.
Is there a way to determine which DNS server the users pc is querying for the request other than running wireshark etc..?
NSlookup brings back the name and address correctly when searching amd it is searching the primary DNS server.

Answer : DNS server not returning requests

If both DNS servers are configured to be reachable, and have different records, you are breaking one of the fundamental precepts of DNS, in that all records must match - the reason this precept exists is because if one DNS server is busy, the secondary DNS server responds with the answer.  Thus, if the records differ, results will be different, which is what you are seeing.  What you'd need to setup is an internal DNS that servers the internal IPs of the site and point your clients to that DNS array ONLY, and then setup an external DNS array that serves the internet facing IPs that internet DNS servers will query for lookups.