It's just like face recognition, it's hard to describe, you just have to study it.

Engine location used to be a good place to start. You need to look at pictures. 727 (3 in the back), MD-80 (2 in the back), 737 (2 on the wing).

Nose contour is another way. 737 have the pointy nose with eyelid windows in the cockpit. A320's have a more roundish nose with no eyelid windows. Among wide bodies, look at the Airbus windows, they are tall and angular vs. the 777 and 767.

DC-9 vs. MD-80. That's harder. Look at the length, shape of the tail (graceful line in the DC-9 versus and angle near the top of the vertical stabilizer in the MD-80).

Odd planes that are no longer flown, such as the Dassault Mercure, Vickers VC-10, Convair 880 (Elvis' plane), VFW-Fokker 614, Trident; just look it up on the internet.