Just renewing my FAA CFI rating and interested to see that the authorities have imposed definite obligations on flight instructors when training "aliens", namely verifying their registration, immigration and security status. Seems a lot of responsibility. And a trial flight is a training flight to me, we do EOC 1.
I do quite a lot of these so-called TIFs (not in USA but elsewhere) and recently had one which I wasn't sure about. I wouldn't go as far as calling the cops but I think we need to take precautions (I checked his ID card, which I've never done before, and in hindsight I can't see how that would help, and made him leave his bag in a locker) but he was a big bloke. Turned out to be charming and, I'm sure, harmless.
Had another once turn up who was completely plastered. We didn't go flying.