Instructions
Type the name of your antique into a search engine, adding any descriptive terms such as "hand-painted," "large," or "signed," as appropriate. Go through the resulting hits and click on those from online stores. These will give you a useful top end valuation for the kind of item you're interested in, as a dealer with overheads to consider will usually ask the maximum the market will tolerate.
Use the same search terms as before, adding the word "auction" to bring up results from online auction sites and from the online catalogs of traditional auction houses. Browse through several live, unfinished lots to find some with active bidding. These end prices will be an excellent guide to the value of a particular antique.
Turn to the online catalogs to find the auctioneer's estimate, an educated guess as to an antique's value. After the auction is finished, the online catalog is usually amended with the individual items' final prices, which are one of the better indications of an antique's market value.