I don't believe you can't conclusively tell about half of the fleet from each other with a exterior trait larger than the reg #. However you can conclusively tell some subfleets apart from each rest. One major difference in 752s is in the exit doors. There's two types - one with 8 doors and no over-wing exits, and one with 6 doors and 4 over-wing exits. Here's my 90%-sure breakdown.
--if you see an aircraft with 8 doors and no winglets, you're seeing a PMNW 5500 series 757. They're all older (from the '80s), so they reportedly will never receive winglets. So unless Delta acquires another 752 with the same two traits, these aircraft will be set apart.
--if you see an aircraft with 8 doors AND
winglets, you're seeing a PMDL ex-TWA 752. Delta is in the process of acquiring another used 752 with the same two traits (N624AG).
--if you see an aircraft with 6 doors, 4 over-wing exits and a small dome on top of the fuselage just forward of the U.S. flag, you're currently
seeing a PMDL ex-song aircraft. I believe the dome is for the live TV
onboard. I stress the word "currently" because as AVOD is installed on PMNW aircraft sometime in the future, this difference will no longer be valid if the system is the same.
I think that covers it, but with such a large fleet, and differences I may not know about, I wouldn't set it in stone. I believe the rest of the 752s all have the same door configuration (6 and 4), no TV
dome, and random yes/no winglet installations.
[Edited 2010-12-23 21:47:10]