Just look for the jerk off who wants to blow stuff up. Like you can always tell who is the spy in a Militia. He is the one with all the automatic weapons and explosives.
I like to blow things up.
I've found he is the one that wants to purchase a weapon of an illegal barrel length, or an unregistered/homemade suppressor.
Federal agents don't have all day to wait for you to do something illegal. They have bosses to answer to.