I can decide if a movie is going to be crappy based on what the voice over actor says. When he says stuff like "Best comedy since American Pie" or anytime they compare it to another movie, it's going to suck. Or when they say "This film is produced by the same people who produced Lord of the Rings!" then I know it's gonna suck just based on that.
You know it's really really gonna suck when they show people supposedly walking out of the theater after watching it talking about how great it was.
if any of that stuff happens in the trailer, it's guaranteed suckage.