All jokes aside, when it comes down to it, MDMA has very distinct physical side-effects. If you know how to recognise them, and someone in your company is peaking hard, it stands out like dog's balls. If you're peaking your guts out, then there's no way in hell you're gonna pass for someone that's straight, or even tired... you can't do much about the pupils, the sweating, the jaw tremor, eye wiggles or heavy breathing, because the drug effects are very strong. Couple that with the fact that you can hardly see straight, you want to hug everyone and you have no inhibitions at all (read: no filter between brain and mouth; brain in neutral, mouth in fast-forward)... and everyones gonna know you're off guts.