Verifying assets for modding (for an online game)

How would one go about verifying the client’s assets with a server?
You see, I’m making a little online game, and if you re-name the level0-1-2-3-etc files,
the client would load the map you replaced it with.
This is of course something with the engine, but is there a way to check their clients for modding?
(I currently do have it set up to check it’s version with a server, but not the files of the client of course)

If you want to make sure the server and client are using the same map, compute their checksums and make sure they’re equal.

http://www.codeproject.com/Articles/23843/Calculating-MD-Checksum