These are by no means all encompassing...but should help....don't tell anyone you use these, they'll think you're a cheat...so let's start big...

1) B747 - Hump on top (unmistakable), 4 engines
2) B777 - 2 engines, long wide fuselage, main gear has 3 pairs of wheels each side
3) B767 - Looks like a slightly smaller 777, main gear has 2 paris of wheels each.
4) A330 - Looks similar to a 777, but has small winglets and "droopy" wings. The cockpit windows on airbus are more distinct from the nosecone than on Boeing AC, 2 engines
4) B757 - 2 engines, much more narrow than the 76, 2 paris ofwheels on main gear
5) B737 - 2 engines, short fuselage, 1 pair of wheels on main gear
6) A320/A319 - looks like 737, but taller off the ground, distinct "notche" from nosecone to cockpit windows
7) B707 - 4 engines, oops

Also, look at the tailcone, or below the cockpit windows, the manufacturer normally labels the AC (ex. 757-300). so that will help you learn as well.

Practice and give it time. Pick out things you notice easy. It's no different than knowing the difference between a Yugo and a Mini...well that's more difficult...

As for resource....www.airliners.net........its great