DC-9

I know the DC-9 is not really a military aircraft, although it is used by a few countries including the U.S. in military roles. Can anyone help me figure out how to determine different DC-9 versions when I see them. Is there really anyway? What would be the most common version I would see. I recently got back from a trip, and I saw just tons of DC-9’s at the two airports I flew into. Can someone give me a run down of all versions and even include the MD-80/90 series and the 717 if you like. I know some about the aircraft, but basically I just can’t tell them all apart. I mean I would tell you the difference between an MD-90 and DC-9 Series 10, but still…. Help is most appreciated.