To identify stoneware pottery marks, search for a mark on the bottom or inside of the pottery. If the mark is not readable, try a different light source, such as sunlight or a bright direct light at an angle, to search for any impressions. Alternately, create a rubbing by placing paper over where the mark is supposed to be and rubbing the point of a pencil over the area to try and get an impression of the mark.
Once the mark is clear, search an online database or collector's book for the pottery maker. Include the whole mark in the search criteria, placing quotation marks around it to ensure that the results include the exact phrase or lettering. If the search yields no result, try it again without the quotation marks, in case it is partially identifiable.
Another way to locate information on a mark is to reference a well-known auction house's website. Some auction houses provide free evaluations of stoneware, as well as an estimated value for auction. Look through the auction house catalogs for past sales of pottery that has the same mark or resembles the piece you have. Check antique dealer sites and past eBay auctions for pottery made by the same maker to learn more about the maker or get an idea of the value of the piece.
There are several books that identify American or European stoneware. Choose a book based on your needs, as some books are more specific than others. Many good reference books are out of print, so find a resource that sells used reference books, or go to a local library branch to find any possible copies.