Instructions
Turn your piece over and look for an identifying mark. If you have a piece made after 2000, the mark will be a blue cursive "D" with an "A" attached. Inside the D is a picture of a standard four-bladed Dutch windmill. There should also be "hand painted, Delft Blue" written below the mark. If the piece was produced between 1876 and 2000, the mark will be a large blue "V" with an "O" on the left upper arm and a "C" on the right upper arm. There may also be a crown symbol that denotes "royal" Delftware. Also, "Handpainted, Delft Blue, Made in Holland" will be written below the crown.
Look online at Old and Sold to identify marks from pre-1876. This resource will show you the authentic markings from all of the Delft manufacturers in Holland before the establishment of the De Porceleyne Fles company (the only company to produce Delft between 1876 and 1914 and the first to establish a continual mark system).
Purchase a copy of "Royal Delft: A Guide to De Porceleyne Fles" by Rick Erickson. This is a must-have resource for identifying Delftware as well as a fantastic historical text. If you still are unsure of the authenticity of your Delftware, go to a professional.
Take your piece of earthenware to an antique dealer or appraiser if you have found a Delft mark. Just because it has the mark doesn't mean it is authentic. Holland doesn't have any laws on trademarked items so several copycat pieces are made in China and carry the same markings. The true test of authenticity is the quality of the piece.