Find out how many hours its been ON - as plasmas have a life expectancy of around 30k hrs - yeah look for discoloration, screen burn, anything that looks outta place really - dpends really where and who u buying it from too.
What plasma? Has it had a Corporate or Consumer life? Inspect it for stuck and dead pixels? Has it been running at max contrast and brightness? Look for permanent after images (screen burn), Look for discoloration!
I'd want to run a test DVD like DVE or AVIA from a DVD player before agreeing to purchase.
Also talk to the manufacturer about service if its an old model.
aligamer - they dont STOP working at 30,000 hours - 1st and 2nd generation panels simply produce around 50% less light after about 30,000 hours.