I made a little AK identification guide, using the parts of the AK that I commonly use to tell the difference. I got cocky on the AK47/AKM, and tried to identify the receiver type, which is probably wrong. Of course, this applies only to Russian AK's, foreign variants will be a little different.
It includes the SVD also, although it wasnt designed by Kalashnikov.

Note, though its not clearly defined here, the the dustcover/upper receivers on AKM's and AK74's are ribbed. This, for the most part, is the easiest way to tell them apart. That, and the muzzle break (or lack thereof), front sight, and dimple above the magazine well (the AK47, since it has a milled receiver, has a large, rectangular depression, while all other AK's have small, oval depressions, as the receivers are stamped).