I don't follow explorers very closely but here's what I know...
Gibson seems to constantly change/rearrange pickup combinations on their guitars and there's a HUGE difference in the sound. That's probably the biggest thing to watch out for. I believe Explorers have been sold with a variety of pickups, typically you will find 496/500 (HOt Ceramic) or Dirty Fingers (Very Hot Alinco), pickup combinations. I'm sure you will find other combinations on custom, special editions or reissues. Dirty fingers were comon in 80s and 496/500 are now standard.
Variation in the wood of the body - Alder, Korina (original standard) or Mahogany (current standard). A few with maple tops - mostly custom or special editions
Tremolo vs. Hard tails - Usually Kahler
Cosmetically, they also have several finish levels which can impact he value quite a bit. "Faded" or "Worn" finishes tend to be a bit thinner and less glossy. They often retailed for quite a bit less because Gibson is able to avoid a few labor intensive finish/sand/polishing steps. No reason to avoid these if the price is right, I have a "faded" Les Paul Studio that I'm really happy with and didn't pay a fortune for.
Watch out for Explorer II's they are laminate body junk from the 80's and not worth much.
Hope that helps.