One method of appraising a bicycle is to use the bicycle value guide on BicycleBlueBook.com. The website has data compiled from years of transactions and gives an accurate valuation of a bike based on year, model, manufacturer and condition.
Another way of appraising a bicycle is by having it appraised by a professional bicycle retailer. Appraisals usually entail a fee of about $20, as of 2015.
You can also appraise your bicycle by using eBay. EBay includes information for products that have been sold over the last 90 days. Simply search for the bicycle's model, while including the year and manufacturer, and click on Sold Listings. The website shows models that have and have not been sold, along with their prices. Research all of the listings, and compare them to the currently owned bicycle to determine its true market value.