Regarding the 732, its easy because of the long JT8D engines.
Concering the 733/734/735: if it has 2 overwing exits, its a 734, I can distinguish the 733 from the 735 by counting the number of window between the foward door and the windowless airframe a bit before the wing, the 735 has less windows as it is shorter (the 735 is approx the length of the 732).
On to the 736/73G/738/739, its all a matter of length, the new family has the larger tail, as stated above, the 736 is similiar in size to the 735 and the 73G is the same as the 733, so the window count approach works (also there are very few 736s around so its easy), the 738 and 739 are simply much longer.
Concerning the MD80, its hard since the 81/82/83 are all the same externally and just about the same as the 88, the MD87 is shorter. The thing with the MD80 series is the tail cones, some aircraft have flat cones and other have the pointed ones, and it seems to be quite random, depending on when the aircraft was manufactured or it is was retrofitted, and not which type.
I thought this was as easy question: what is the difference between a 737 and an MD80 - just kidding, regards.