If you actually want a paid service, there are plagiarism detectors like Fossa and Codequiry. Although in my opinion, code review should be enough to catch any "accidental" incidents of plagiarism, the differences in writing style should make it very obvious when the employee has copied something. That of course probably won't apply if you suspect the employee is intentionally changing it around to obfuscate the origin of the code, but it seems that wouldn't be the case if they were just committing the output straight from a neural net. But automated scanners probably won't be able to catch those well either -- the way to catch that would be to make them do pair programming a lot.