They're so damned easy to spot once you know. I started out catching them in our college bars.
Had an agent of some sort follow me around the airport. He was clearly trying to look nondescript but his build, hair, clothing, demeanor, boots, all screamed FEDERAL AGENT. I only noticed him because I saw him a second time, which is a little odd given most people in an airport get where they're going and park themselves. After watching him a bit he was actively avoiding my gaze. I guess the military backpack, combat boots and long hair made me look like a terrorist?
I fucked him with a little, doubling back and making him follow, watching him in store reflections. LOL, he stopped at a closed store to watch me in the glass. Kicker is, it was a woman's store and there was nothing interesting you could see from the concourse. And worse, he stood like a mannequin, utterly uninterested, obviously didn't even know what store he picked. Guess he picked up on me fucking with him and bailed. Never spotted another tail, but who knows?