It's almost impossible without outing yourself. I've had meet-ups with people from this board at conventions which was awesome. But, we had already been communicating here so its not like we were meeting for the first time.
The programs are so long and boring, lots of good JWs end up doing other things, wandering around, leaving early, sleeping, that you really can't read into those behaviors.
I wish there was a secret sign that a PIMO could use but most people are PIMO for a reason (too much to lose at the moment) so even if a sign was worked out, I'd be paranoid to actually use it.