Recognize a 747-100 from a 747-200 is a nearly impossible thing -
I an generally 99% correct when saying it is one or another, not by the looks, number of UD windows, or UD doors, but merely by what airline or registration... and then here or there, I am wrong. Even 747 pilots cannot be sure.
For me, the only certainty, is looking at the gage of the center wing tank on the flight engineer's panel - and read its capacity, all the rest is basically the same and could either be 100 or 200.
If there were 75 airlines ordering 747-100/200s, there are basically 75 different 100/200 series. The only real "experts" are airplane spotters, certainly not the pilots or mechanics...