Delftware is classic earthenware produced in Delft in the Netherlands. Each piece is hand painted in blue on the stark white clay. Higher end pieces may be adorned with metallic edging or be finished in tin glazes. Although the most-sought-after pieces were produced prior to 1750, even the inferior artistic works between 1750 and 1900 are quite valuable. Delft primarily dealt in plates and vases but for about 200 years prolifically produced tiles. A conservative estimate puts production near 8 million individual tiles.
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.
After World War II the tourist industry bloomed in Holland and several Delft manufacturers began producing "tourist" quality pieces. They show little of the artistry found in true Delftware and exist in large quantities, making them far less valuable than the high-quality, hand-painted, pieces.