Leaks, whistleblowers. Some circumstantial evidence will also do if there's enough of it. Like having hallucinated parts of code that do absolutely nothing, and can't be explained as e.g. leftovers from a refactor.

> They would have to get their hands on your prompts

Unless you are running a local model, your prompts are almost certainly logged by your inference provider, and would only be a subpoena away?