The theater size might give you some indication. The smaller the theater, the less likely they will have tickets for Opening Night.
But from what I heard from people who have been (I have friends who won If/Then ON tickets in a sweepstake) it's like any other performance, really. Not even sure they have a curtain call speech or anything. The only thing is probably that there will be celebrities in the audience, but if that's what you're after you can go to the red carpet beforehand. I did for OTTC last spring, it was just a bit of fun to see who all turned up.