To identify Delftware makers' marks, look up the mark in a book dedicated to Delftware markings, such as "Discovering Dutch Delftware: Modern Delft and Makkum Pottery," by Dr. Stephen J. Van Hook. Another option is the book "Royal Delft: A Guide to De Porceleyne Fles," by Rick Erickson, which focuses only on Royal Delftware.
"Discovering Dutch Delftware: Modern Delft and Makkum Pottery" includes pictures and illustrations of marks for a large amount of Delftware. It also has historical and production information.
"Royal Delft: A Guide to De Porceleyne Fles" includes color and black and white pictures. Many of the black and white photos come from original sales catalogs. The book also includes information on the rarity of certain pieces, their original prices and their current values.
Delftware comes from a factory in the town of Delft, which is in Holland. Delftware can be difficult to identify, as the markings may be a factory name or a person's name. If the markings are a person's name, its typically the name of the factory owner or an artist at the factory. Also, the Netherlands don't have strict laws on product labels, so labels may be inaccurate.
Factors that affect the value of Delftware include the piece's age, size and condition. An appraisal is one way to determine a piece's value, although the owner can also check online marketplace listings for the same product to get an estimate of its worth.