Choose or be Chosen?
If you invite (even the average) person up to the flight deck then I guess the odds of this being Terr O' Wrist are somewhat slim. If they invite themselves and you agree then the odds are significantly higher.
Stand at the side of the road long enough hitching a lift and you WILL allow a sicko to choose you, if you approach someone and ask them, your chances are around average, depending on your judgement.
We are prepared to trust the captain's judgement on so many things, why not on this?
(But follow Groucho Marx and say no to anyone who actually asks!)