This post regarding the MD-80's came up awhile ago, so you really should check the archives.
Anyway, just a summary:
First of all, there is no such thing as an MD-80 per se. They are all the latter designations (-81, -82, et. al.)
All of them with the exception of the -87 are visually indistinguishable. They differ only in areas of weights, engines, avionics and so on.
The -87 is easily recognized because it is much shorter than other MD-80 family planes, It also has the extended tail fin a-la the 717 and MD-90.
As for UAL cargo, I haven't a clue.
Hope this helps.