I can tell within 5 seconds of walking into a room which patients are on chronic narcotics, "disabled", or are just plain nuts, etc... Pattern recognition works. Same with police work I'm guessing.
An RN in my family says that doesn't matter in the ERs she has worked. The doctors give them the "free" pain meds anyway.
My wife has worked at a drugstore. They line up in the drive through every morning to get their "free" pain meds.