They can pretend to sell you drugs but they can not allow illegal drugs to be released onto the street. They get in big trouble for that. So a good way to know if someone is an undercover is to ask them to bring you a joint. If they can't or wont then that would raise a flag. All evidence must be accounted for so if you sell someone a small bag ask to buy a nut back at any cost. If they refuse beware cause your probably about to get busted.