Well, if it's REAL MDMA..then you'll know because you feel a mental high as well as a body high. Extreme happiness and love for everyone/everything, acceptance towards yourself and people around you, your body will feel sensitive to touch like an orgasm. You'll notice that your jaws will start to lock down hard, or start to chatter like you're cold, but just chew gum for that. You'll see the world in human HD brightness, clarity, trails of lights, especially if you get nystagmus (eye wiggles).
What you'll most likely get if you're in the U.S: A body high that starts to build up once it starts hitting you, almost getting to a nice peak for about an hour. It's kind of like masturbating...and right when you're about to cum...some asshole comes around and snatches your hand away and doesn't let you finish (aka: methylone).
EZ test your pills, home slice.