Well, driver, it's your lucky day. I just stumbled across your post. I am not a lawyer but I watch a lot of TV (starting with Starsky & Hutch thru Law and Order:We Ran Out of Names). All you have to do is ask the guy if he's a cop. If he says no, and you get busted, the case gets thrown out cuz he lied to you. If he does do some of the drugs with you, and you get popped, I would place him under a citizens arrest, just to turn the tables. Maybe he'll get scared and just forget the whole arresting you for soliciting the sales of a controlled substance thing.
BTW, this was all tongue in cheek. Just thought I should clarify that, in case your head isn't as clear as it should be now.