Most guys selling drums aren't going to want to have you completely disassemble them.
You're going to have to mostly go by what you see from the outside - scratches, dings, rust, general condition, and so on.
Approach it right and you'll probably be able to pull off a head or two without getting the seller ticked off,
and check the condition of the bearing edges, and check for ply separation.
You can probably tell quickly if a set has been cared for, or if it's been neglected or abused.
If you know someone who's been around buying used drums before, it might be helpful to bring them along.
Unless you want a project on your hands, just walk away if you see any signs of abuse or neglect. Things like rust, damaged parts (other than minor shell dings), missing parts, peeling wrap, etc. If the heads are worn, that’s likely fine since they’re easily replaceable. But if they look beat to within an inch of their life, then I’d definitely take off the head(s) and inspect the bearing edges to make sure those aren’t beat to hell as well.
Other than that, if they appear to be in good condition, they most likely are.
Are you buying from an individual or a store? I ask because if an individual and you go to them, look at their other stuff. Not thoroughly, but look. If most of their stuff is unkempt, they probably didnt put too much care into the drums either. If their stuff is well taken care of, chances are the gear is too.
This is a really douche bag way of looking at things, but there is truth to it. And if you dont know WHAT you are looking at, at least know WHO you are looking at.
If you don't know how or want to repair used drums or equipment DON'T BUY IT. Simple as that. When I buy used drums, I negotiate the price to reflect potential costs of bringing the drums up to playability. A 1940s Slingerland Radio King for $700 is not worth that price if the strainer is questionable, or if it has an after market strainer with or without new holes drilled. Also if the original engraved double flange hoops are not included or it's missing the internal tone control. All those replacement parts are going to cost a lot to replace, meaning you're paying more than if you were to find a Radio King that doesn't have those issues. If you're looking at an Acrolite and find one for a couple hundred dollars (at the most) and all it needs are new heads, get it. Always look at the cost of bringing the drum to playability when negotiating the buying price. Message me if you have any questions. I've been doing this a long time.
If you do happen to be a little diligent, bring a drum key. Here’s my story:
I went to one of the Malls in the Springs to meet up with a guy who was selling a Rogers holiday in Mardi Gras (I somewhat regret not purchasing).
Price was non negotiable but he had on his 12”
or 13” (yes was a 5 piece w/snare) coated ambassadors top and bottom. I had no key and neither did he nonetheless, I chalked it up to a possible damaged shell but if I had a key would have been able to look in, or it could’ve been double headed with coats.
I asked him about the shells and he gave me a backstory of this one shell and the bass, being found outside by a dumpster in the Vermont rain.
Nonetheless a key is very useful to get an onsite inspection if u know what to look for as far as bearing edges, wrap jobs, inner plies, etc...