The one quickie indicator is to look at the rear drop-out like Danny suggested. Good bikes have good quality dropouts and cheap bikes have cheap (often simple stamped steel) dropouts.
If you find a bike with good dropouts, you will probably also quickly recognize that the rest of the bike is high quality. Just look at a lot of bikes. Also, go to the bike shops and look at bikes from cheap to upper-end. Pretty soon, you will be able to tell the difference between cheap and good bikes.