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.