How to tell if the record is worn out?

Hi
On my recent trip to WFMU annual record show i encountered a problem that i've never dealt with before.Out of 50 records that i bought there 30 turned out to be worn out to a point where you can't even listen to them.Visually they look perfect though as if someone carefully played them from the beginnig to the end a 1000 times on a bad turntable,not a single scratch or fingerprint.I don't know if i's just a bad luck or since the show became annual instead of twice a year maybe many dealers do not care about returning customers any more?Is there any way to tell if a record is in bad condition?Maybe i should bring a big magnifying glass with me next time,the problem is:i don't know what to look for.Any opinions on this one?