Your best bet is to check completed listings on Ebay.
When you do a search for specific lens, you can check a box to limit the search to completed listings (you'll need to be a registered Ebay user for this function now). Then, look at what they sold for. ;-)
Checking prices at dealers is another way (of course, a dealer that has a return policy, limited warranty, established reputation, etc. is likely to charge a bit more). These vendors all sell used gear:
http://www.keh.com
http://www.bhphotovideo.com
http://www.adorama.com
Jim C.