Airbus 319/320/321 series planes are distinguished by the "wingtip fences" Shareholder described, whereas the narrowbody Boeings (737/757) don't have this. I find it difficult to distinguish the planes by just looking at the tails (unless they are side-by-side). Also, A320's have single piece wing flaps, while the B737/757's have triple-slotted flaps. The 757 and newer 737s (-300 series or later) have engine pods that are mounted kind of forward of the wing and do not hang below the wing as much as the engine pods on the 320.

The A330 and 340 are nearly identical except that the 330 uses two big engines and the 340 uses four medium engines. Both have the upward pointing wing tips ("winglets"). The Boeing 767 has no winglets and has a narrower fuselage.

FewMiles..