In general, undercovers aren't interested in busting individuals who are on drugs- they want to stop the selling. I've yet to see an undercover pretend to sell- they almost always try to buy. We probably get approached 5+ times in clubs and raves when we go out from people wanting to buy MDMA in LA even when we aren't rolling- most of the time, I'm pretty sure they're just genuinely wanting to roll. One time a guy who looked uncomfortable, over 30, and kind of a meat head approached my boyfriend and kept bugging him and bugging him for pills, he finally said "dude, I just want what you're on, sell me that?", and my boyfriend goes "OK, I'll go to the bar and buy a gin and tonic, and sell it to you, OK?" LOL, the dude finally gave up. I think my boyfriend tried to hug him good bye, but the guy wasn't interested in hugs, LOL.
Hey, are you going to Monster Massive? Can I buy pills from you there? I'm gonna want what you're on!
Seriously though, as for the question of the original poster and others, in my experience, the only thing you need to do to keep yourself safe if you want to share that you are off your ass is identify those who are OBVIOUSLY rolling and communicate about your current state only with them.
I.E. If sober-seeming people ask you what you're on or if you're rolling, just give 'em that shit eating grin of joy that only someone rolling at a rave can muster, slap 'em a high five, and go back to the music and/or give 'em a big sweaty hug and party with them, cuz they need the good vibes. :D
If someone is OBVIOUSLY rolling (massive pupils, sweating profusely, chewing their cud) then it's probably OK to share that you're rolling your ass off. Honestly, it's probably OK if they're dead sober, since "ROLLING" could mean anything and probably isn't enough to incriminate you for shit. And, I don't know about elsewhere, but here in SoCal, it's known that 80% of the people at raves are rolling and the security does NOT give a shit and definitely won't kick you out for it. Shit, I have been at a rave with my brother who TOLD one of the security guards we were with that we we on MDMA (I almost killed him) and the guy just thought nothing of it and continued the convo with my brother.
The KEY is simply to tell people, when asked, that you don't know where they can get more pills. Period. If you don't know them, you don't know where to get pills. This is the safest strategy. That, and not buying at a rave.
However, if you MUST buy at a rave, which is high risk and should be avoided, but hey, you only live once and sometimes it happens, this is my strategy.
First, step back from the crowd and observe. In general, you'll see the obvious looking dealers (they look a certain way depending on where you live. Here in SoCal, it tends to be a flat-brimmed baseball cap, a bandana around neck / across face, and generally tweaked out appearance, gangsterish / cholo clothes). Try to watch a few transactions go down successfully (i.e. someone comes up to them, very subtle / low key exchange of pills & cash, then the other person leaves without getting arrested). If you see this happen a few times without the buyer / dealer disappearing, it's probably reasonably safe to get pills from that person.
An alternative strategy, one that has been extremely successful for me (but only if already rolling and wanting more pills) is to ask someone who is clearly rolling balls if they are rolling. When they say, "Fuck yeah" revel in the shared moment of enjoyment. Become their friend, and enjoy the music. Once you have formed a GENUINE bond, ask if they know anywhere you could snag another pill. This usually works as they trust you and you've bonded, and you're connecting with them on a level beyond just going up to a random stranger and asking for a pill (you ARE, aren't you?!). If they say no, no worries, just keep enjoying the music and their company since that's what it's about anyway.