Is there a way to prove that your computer program code was stolen?
The only way I've thought of is visual inspection but unless it is open-source I don't know how one could get access to the other party's source code.
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Subpoena the code:
The only way I can imagine to prove IP theft in a scenario where you are unable to access the infringing source code is to subpoena the code. I am not familiar enough with US civil procedure to cite you the relevant law, but here is an article where some people have had their source code subpoenaed.
For UK law, you can subpoena a witness to produce documentation (in this case source code documentation) (Civil Procedure Rules 34.2(1)(b)), including for the purpose of obtaining evidence before trial for the trial (CPR 34.1(1)(b))
But of course, this all applies only after a suit has been filed.
If you feel you are likely to have a claim, a reverse-engineering firm may be able to help you determine if your code was stolen. You would need to provide your source code and the accused product, but it is possible. In fact this technique is used to prove non-compliance with Open Source licensing obligations, even in compiled applications or embedded firmware.