Not always ture on the DC-9 is smaller then the MD-80s.
The DC-9-50 is about the same size as the MD-87, and the 717-200, is the same size as the DC-9-30.
a good way is the the MD-80's hav a larger JT8D-200s then the older JT8D on the DC-9.
Also a better way, all MD-80s except a few like some Continental's, have a V-shape point at the back of the MD-80 (like the 777s), as the DC-9s have a a rounded point in the back.
Compare them to the pictures here in Airliners.Net to bet a better understanding.