They have been at the Arizona shows regularly for quite some time. The ones doing PR duty wear black polo shirts with ATF embroidered on the breast. The rest of them are undercover and you can bet they are hard to spot. I ws going out to my car and they were busting a couple dudes for something as they reached their car (about 3 cars down from mine). There must have been 8-10 agents with their badges hanging around their necks, there were even two females. What I noticed was they were a mix of white and hispanic (lots of that ethinic in Az) and they seemed to fall into two visual types...
1. Tall (like 6'5" tall) clean cut, late 20's-mid 30's athletic type.
2. Dirtbag biker material, unkempt slob looking - these you would have a very hard time spotting in a typical gun show crowd - they look like many of the bubba's and low life so often seen (no reflection of current company... just say'in).
3.Same for the females - they looked well kempt but a little on the lower end of town side- like a normal gun chick, but their badges sure were bright and shiny.
One tip off is they almost always work in pairs when trying to get someone to screw up. One does the talking and the other is the silent buddy observing everything.