How to check a Firmeware or .exe to be safe ?

[o_e_l_e_o]

If he has indeed altered the software, then it will be trivial to notice the difference between the compiled binaries and the pre-compiled; they will give different hashes.

As PowerGlove points out, of all the open source wallets out there, only the minority are actually reproducible from the code provided. Take a look at https://walletscrutiny.com/ for example, and see that the majority of open source wallets are either unreproducible or throw errors and do not compile at all.