First hand report:

This morning, at the Orlando International Airport by the Delta gates, two plain clothes federal air marshalls pinned and took into custody a 20-ish, socially challenged, caucasian man who was causing a VERY LOUD DISRUPTION.

Until their quick action (they quieted the guy down very quickly and moved him into a corner away from other passengers), it was completely impossible to distinguish the marshalls from any other passenger moving through the terminal on business.