How to know a given file is a valid bitcoin wallet? If I have many files on my hard drive (possibly with different names/extension), how can I test which file is a valid bitcoin wallet.dat ? 

Maybe there is some string that exists in every bitcoin wallet ? 

I guess I am asking about the format of the data inside the wallet. 
If wallets are different depending on the bitcoin client, then I want to know about bitcoin core.

Edit: I forgot to mention but these files WERE on a Linux OS. So there should be some solutions.
On Linux, you can use the file command. If the file is a Bitcoin wallet, you should see

wallet.dat: Berkeley DB (Btree, version 9, native byte-order)


(It will always say Berkley DB version 9, but the filename might be different.)

file is also availible on Windows: http://gnuwin32.sourceforge.net/packages/file.htm

How does file tell that it's a Berkley DB file? See How did file identify this wallet file?