How do you spot an overplayed used lp without obvious scratches etc

Agree with others that you cannot tell from an eyeball inspection. I’ve had records that looked pristine that were trashed, and suspicious looking records with surface marks that have played beautifully. Also agree that deep cleaning can salvage some. Another thing I check is whether the record is warped. You really have to put it on a turntable and spin it. If I am buying online, I engage the seller in a dialogue to suss out as much as I can. For more expensive records, with a return policy, it isn’t worth the time/effort for either party to sell a mis-graded record. For expensive older records, I insist on play-grading, and although that’s not foolproof either, it is another precaution. On balance, it it is still a bit of a crapshoot, but well worth the effort.

As stated, it's a crapshoot. Mishandling: spindle hole damage, ring wear, lack of original inner sleeve, etc. are warning signs. But, I think the biggest risk is vendors who know the tricks and dress-up old records by polishing the vinyl, cleaning the jackets, replacing missing inner sleeves. I see this at audio shows where hucksters are asking very high prices for "rare" used and nos records, many of which are available new or re-issue from legitimate sources. Some show vendors offer headphone listening, but crank the treble down to hide audible hiss. Also, warp is a real risk on sealed nos stuff.

My personal rules are buy lots of cheap (> $1) stuff at thrift stores and garage sales  and expect to re-donate 90%. For moderate stuff ($5-$10), deal with reliable local vendors who will accept returns. For audio show vendors, leave your credit cards at home.

When I'm selecting records to buy at thrift stores or garage/estate sales, I always insist upon taking the records outside and looking at them in the sun. The fluorescent lights in record stores and thrift stores will not reveal defects in the records ... but the sun is like putting them under an X Ray.  Also, a good thing to have at home is a good LED lamp. They let you see what's on the record too. 

Bottom line is that the naked eye can't see microscopic groove damage from worn out stylii, only dirt and surface scratches. Even then, if your stylus geometry is different (van den hul vs elliptical) you may play a different height on the groove then where the wear is from the prior bad stylus used so that the record sounds fine on your machine and bad on your friends.

Good LED flashlight is excellent for seeing even very small hairlines. 120 lumens is enough. As for the OP's question, I agree with the others, you just have to clean and play it unless perhaps you have a powerful magnification device.
davide256, excellent point about stylus geometry, now I know it, thank you.