There are several ways to spot a 737. And you can also tell the generations. The -100s and -200s have long cigar-like engines under the wings. The -300s, -400s and -500s' engine cowling is not round, but tapered in the bottom. Other than the -100s and -200s, the vertical stablizer (the tail fin) is not a triangle, but has a leading edge with a shallower slope in the forward portion where it attaches to the fuselage.

The 321 (usually) have 4 doors each side. All are seperated. The 320 also have 4 on each side, but the middle twos are together, over the wings. I believe the 319 and 318 have only one exit over the wing, for 3 total each side.