There's really only two criteria I look out for:
1. The trailer/previews and commercials: The two and a half minute trailer for a film can be a deal maker - or breaker! - for me. Does it draw me in and make me intrigued to see it? If it's for a comedy and it makes me laugh, then that's a good sign. If it's not for a comedy and I laugh, then that's probably not a good sign! Some movies you can tell they're going to be garbage from their previews. (The Bounty Hunter is a great example; it looks like pure sh*t). Oh, and here's a little tip if you already didn't know: whenever they have "real people" talking about how great a movie was and how "you should take your whole family to see it!" or "I would see it again!" then that's an indicator that a movie is a stinker and they could only get praise from the general public because the film got slammed by the critics...
2. Rotten Tomatoes - RT is a great way to determine what the majority of critics think about a certain film. Sometimes reading just a couple of reviews doesn't really help because I've loved some movies that a couple critics hated and vice versa. With RT you get a cornucopia of film critics' reviews which they compile together and come up with a percent on how "fresh" or "rotten" it is. If a film is on the fresh side, I will consider seeing it (if it's something I find interesting); if it's REALLY low on the rotten side, there's no way in hell I'll see it and if it's in the middle, I'll make my best judgement.
So there you go! That's my secret!