Doctors, please pay attention.
It's really simple actually, to spot a liar. You're not as bad as you think you are.
Here's one clue: lies never hold water for long.... people's memories generally last about three months. Ask anyone who has to deal with liars long enough. You keep their close company for a month, and you'll see the lie, easily.
However, I know all y'all get scant contact with effing patients these days, so here's what you do. You use those notes you have to keep, by law, and you ask the patient questions which you already know the right answer too. If you don't like the explanation, well, no medical lawyer would fault you (I'm not one, so best consult with them first ;). I know a few though, so if you need any names.... just ask ;)
I can't spit in my family without hitting a lawyer. Not that I'd spit....