Whatever band is currently drawing a big druggie crowd - it was the Grateful Dead for decades, then Phish, then Widespread Panic, now it's probably ICP - you can always expect to find some Feds. They like to focus on acid/mushrooms more than weed, because the punishments are worse. Concert organizers will put together narc-on-the-narcs teams whose job it is to pick out the undercover police and throw them out of the venue. They can ask anyone to leave, but they can't prohibit cops as a policy. One tip I remember was that they said all the undercovers seemed to have fanny packs on - I guess to carry all their cop stuff.
What they were doing at a lot of the busts was using a honey trap female who was scantily clad, without a man, and pretending to be very drunk. She would ask you to point her toward someone who would sell her acid. All it takes is pointing a finger at someone and that makes you an accessory to the sale, which is the same punishment as selling, which is a 5-year Fed mandatory minimum, even for a first time offender, and even if no illegal drug transaction actually took place.