utter waste of time
(which I clearly have!)
....to tell the difference between the smaller ones. Airbus have winglets on the wingtips (to assist efficient airflow over and under the wing). Boeings dont.
No winglets but with one or two overwing exits each side is a B737.
With winglets and one exit overwing is a A318/ A319.
Two overwing exits each side is an A320.
Longer range aircraft have more main exit doors which are bigger than the self help overwing ones. From the side they look like: nose - door - door - wing - door - door - tail.
The A321 from the side looks like the B757 but has winglets. The B767 is a larger version of the B767. The B777 engine is huge and equals the size of the cabin fuselage (and on some airliners B777 is written on its ar$e!).
Hope that helps but as aviate suggests, you have to study piccies or live a/c to get it.