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

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.

While you cannot tell for sure with a visual inspection, there are a few clues that help to weed out some candidates.  First, look for a lot of use of the record, such as a lot of marks on the label near the spindle hole (caused by someone sliding the record around to find the hole.  Another sometimes helpful visual clue is that the playing surface on a well-worn record will sometimes appear grey and not as shiny as a newer record. 

Of course avoiding scratches is the obvious way to avoid bad records.  However, if it is a title I really want, I will accept records with a lot of scratches as long as the scratches are not deep.  I have plenty of used records that sound like they are new even though they have a lot of light scratches; the line-contact profile of my stylus tends to play deeper down in the groove and often avoids surface problems.