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

Simple fact is you cannot tell without playing the record.

I bought a supposedly "pristine" Japanese pressing of Genesis Trick of the Tail from an EBay seller who claimed to have a $10K trurntable and associated arm/cart, a VPI album cleaning system which was used before every playback, etc etc. The surface of the vinyl looked pristine. But on playback the sound was flat, dark, and lacked transient detail. My US Atco versions of the same album sounded orders of magnitude better. Sent it back for a refund.

In another case I bought a 1st pressing of Blind Faith, British pressing. Surfaces where lightly scratched and scuffed profusely. Sound was pristine. So looks don't mean squat, unless you know the owner and know for certain how much the LP was played and on what equipment.

I've been stung too like that.LP looks great at a show.I don't know how to spot a bad one unless play graded.Just last week I was giving my friend some extra Streisand LP's and had what appeared a mint copy of her Broadway LP,which is a good sounding vinyl except this one must had been played on a table that destroyed the soundfield,high end.I have managed to remove unbearable noise on RCA living stereo's and other better sounding labels by repeated deep cleanings manually.For years I had a Fritz Reiner Scheherazade that had unbearable surface noise rendering it not listenable untill I recleaned it manually.Now it's a prized piece in my collection.

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.