Jokes aside, you ask in different ways, including different languages, and the more you test the more certain you are that it is correct. The only way to be 100% certain is to get the developers to tell you.
Almost every post that I read about the subject seems to omit that fact. Ultimately it boils down to a kind of fuzz test and a form of inference over the results. It would be interesting to study the possibilities of quantifying such certainty.
Jokes aside, you ask in different ways, including different languages, and the more you test the more certain you are that it is correct. The only way to be 100% certain is to get the developers to tell you.