<font face="Verdana, Arial, Helvetica, sans-serif" size="2">Originally posted by Human Unit 763246B:
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).</font>
The tailcone is a way to tell those apart though but not 100% reliable.
The DC-9s had the rounded tail cone (and rear airstair). Early MD-80s are similar but many of the latter didn't have rear air stairs and the tail cone is more like a flat blade (similar to the 777).
FWIW, the 727s can be mistaken by some for a Hawker-Sidderly Trident though I didn't think any Tridents are stil flying.
Some other easy distinguishing characteristics:
A300/310 - a long ridge in the centre-rear top of the fuselage.
777 - 3 pairs of wheel per main bogey. Flat "blade" tail cone.