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


I'm pretty good at spotting record defects and scratches that will or won't affect sound quality.
Even with a well done cleaning, you play what looks like a good album and it has simbilence, a rough thick surface noise etc.

how do you spot these from a gem pressing?, 
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I agree with jerroot about used record vendors cleaning their records before they put them for sale in a shop or at a show. 

So besides checking the labels, pay close attention to the jacket.  This will tell you a lot.  Any of that white ring rash around the jacket corresponding to where the record would be inside?  A pretty good sign that the records were stacked for years instead of being properly stored upright.  Walk away!  Also is the original cellophane still on the jacket with the opening being sliced with a razor?  This is a very good sign that the record was properly cared for and played on good equipment.  The original buyer cared enough about keeping the jacket in the same condition as the record.

Last, buy Japanese pressings of your favorite titles whenever you can find them as these have really held up well over the years.  Since it was audiophiles that bought them at HiFi shops and at premium prices, you know that they were well taken care of.   All of my Japanese pressing are still free of surface noise and among the best I have in my collection.

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.