How can I identify Lego sets from a box of parts?
Your best shot is to identify an individual piece and look it up on Peeron, BrickLink or BrickSet. Since these all carry set inventories, you'll be able to find lists of sets in which the parts can be found. Try to start with more unique parts, chances are they are in a few models only.

Once you're sure you've found a model for which you've all the parts, build it using the instructions from LEGO.com or for older models instruction scans PICSL.

If identifying LEGO parts proves too difficult for some of them, ask experts.