There's also a laminated card that they sell in places like airport bookstores which my wife got me a few years back with the major airliners and what to look for to tell them apart... I think she thought she was making fun of my planespotting obsession, but I thoght it was pretty cool :-)

The way I resolve the Airbus vs. 737 is by the little winglets at the end of the wing for the Airbus, and then count window exits to tell between 319 and 320 (don't see many 321's where I am). 737 I tell by the little ridge thing in front of the tail... (at least for the later models)

Dan