Instructions
Look at the head. Tonner Dolls have the words "Robert Tonner" and the year of manufacture stamped on the back of the head or neck. They do not have any serial numbers or other markings. If your doll does, it's probably not a Tonner.
Search Tonner's site. You can use the "Search" function, or browse through their collections. In addition to having the current catalog, they maintain archives of sold-out products.
Bring the doll to a collectors convention, antique shop or doll expert. Being able to see the doll in person will help identify the doll, since the appraiser can check for textures and weight, in addition to appearance.
Post an inquiry on a doll collector's forum if no local doll experts are available. Collectors are often enthusiastic about sharing their knowledge. Include a detailed picture of the doll, and be prepared to answer questions.