Comparative shopping - check past sales on eBay and see how much they sold for. Also, check out Craig's list to see how much people are asking for them.
You can even ask your local bike shop. They will have a good idea of how much your bike is worth. They may even be willing to sell it for you or let you put up an ad in the store.
Best place to sell is Craig's list though. Local shoppers, no fees. I sold my bike to the first person who responded, and that was 20 minutes after I posted the ad. I had 25 people email me about the bike.
what model is it?
have a look at similar ones on ebay, see what they sell for.
why ask me
check the web, ebay, google, cactusbike.com (if u ask there u will get a good answer)