When I buy a used tank I look for flaking rust. Not powdery flash rust but real chips of rust, and pits inside. Never buy one with flaking rust. Also look for dings and gouges on the outside. There is a limit on how deep they can be. I'm sure a tank inspector type will chime in with that info. Remember that a lot can happen between the last hydro/vis inspection. They may have let salt water get in the tank on a dive, filled the tank, and let the tank sit for 8 months...
I've also taken a lot of chances on old 72's and come out good so far. It all depends on how the tank was cared for.
If the deal is good enough, it might be worth the chance without looking inside, but if I have the chance, ALWAYS look inside.