If anyone wants, I could about triple Sentinel Chicken's post, with lots of boring, pointless info details, like the exact weight/range/fuel changes between them, as well as the engine's thrust/temp/weight changes. (I've got plenty of info in my 2 main MD-80 books)
But the basic answer is still, unless you rip open the fuselage and engines to look for minor INTERNAL differences, you will never be able to tell 81/82/83/88 apart, even from only 5 feet away.
(The -87 is shorter, with the extended vertical fin, the -90 has the -87's tail on a body longer than the others, and HUGE engines, and we all know the -95/717)
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