According to the resource website Milk Glass, it is easy to identify milk glass because of its opaque, "milky" or "creamy" appearance. The trick is in determining the age of the piece, because older milk glass is of much higher value.
Milk glass has been in production since the 16th century, but most of the milk glass found in modern times dates from the 1840s and later. Rare examples of 19th century milk glass are considered very valuable, and early 20th century milk glass is also sought after. In the 1950s and 1960s, milk glass became very popular, and there are many mass-produced pieces from this time period. Although they are very attractive, these later pieces are less valuable.
The age of a piece can be determined by the maker's markings, the patterning and the color. Milk glass in a color other than white (pastel green, blue, purple or pink) means that it probably dates from the 20th century; this is when adding dyes to milk glass became very prevalent. Milk glass can be difficult to accurately identify yourself, and if you have a piece you are unsure about, the best course of action is to take it to an antiques appraiser who can help you determine the provenance and value of your piece.