It's actually not that hard to tell - run an MD5 hash on any meshes or textures you suspect. It's like a digital fingerprint. If they match your/someone elses meshes or textures, it's a copy.
http://support.microsoft.com/kb/841290
This utility supports SHA-1, which is even better.
http://support.microsoft.com/kb/841290
This utility supports SHA-1, which is even better.