In shopping for a used Express gun, I no longer even look at those made since the shift to magazine tube dimples. You know how to conduct the basic inspection- outside, bore, what you can see and feel of the action without taking anything down.
Then ask the dealer if you can unscrew the magazine cap first. Let them do it if they'd rather. Start unscrewing the magazine cap, and if you hear a clack-clack-clack, stop, tighten back up on the mag cap and put it back on the rack. That's the sound of the plastic mag cap retainer doing its thing. If you get the cap off and see a toothy plastic insert in the end of the mag tube, put the cap back on and keep looking.
No noise likely means it's an older gun. when you remove the magazine cap, look for the detent in the barrel ring. If you see it, that's a good sign. Push up the lifter and look at the front of the trigger plate assy at the back of the loading port- you should see two light colored rectangles which indicate the trigger plate is aluminum. Look at the extractor- if you see mold marks, it's MIM. If you see tool marks, it's machined. You want machined for an older Express gun, but it's an easy and pretty inexpensive replacement ($8 IIRC) if everything else is looking good, and I have yet to break an MIM extractor so far.
With 870s, the most likely no-sale abuse is bubba-ing from the kind of owner who can't resist taking apart things that were never meant to be disassembled by the owner. You need to know the design well enough to tell when gunbutchery has been committed, and you need to have permission from the dealer to pull the trigger plate assembly for inspection.
Beyond that there shouldn't be any trouble in picking out a good one...
fwiw,
lpl