We advise you to search this forum more thoroughly.
The sticky thread about 2nd hand lenses is just right here, on top of the list: http://www.clubsnap.com/forums/showthread.php?t=311048
Secondly, using Google with '[lens type] problems' will reveal a lot of hits and you can start reading about specific issues with that lens. Should give you enough pointers to look out for. Don't be shy, print out a check list. If the the seller doesn't want to give you the time for checks better walk away. It may also help to get a friend into this deal who is a bit familiar with the topic.