I you are suspicious of a piece, leave it where it is unless the dealer will agree to an expert appraisal. If you have a collection that you plan on selling, be sure to find out what it is worth. There are several ways to do this. The first and probably the most accurate is to put out a few dollars to have your collection appraised by an expert. You could save hundreds of dollars by seeking professional advice.
Price Guides are another good way to check out the value of either an item or an entire collection.
These books list antiques and collectibles by category, often including photos. The prices are either listed beside the items or in the back of the book. Your local library has an extensive inventory of price guides on all aspects of collecting. If you like, there are places on the Internet to check the value of your collection. Online auctions have thousands of items up for sale at any given time.