There are some simple means you can use not to 'prove', but to find out if you want to prove code copying, if the original code is assembler.
1) measure the time from reset release to the activation of pin x. if it is different, the code is different.
2) measure a pulse train or whatever on a given pin, if it is different, the code is different.

Using simple methods like this I would, with access to schematic and code, in half an hour or less be able to say "do not waste your money" or "there is a 99% likelyhood the code is copied"

Erik