Dating to the 17th century, the earliest dolls were simple peg-like wooden objects dressed in scraps of fabric. Porcelain dolls--in fact, dolls with porcelain heads, as the bodies were usually made from a pressed wood-pulp material known as composition--were popular from the early 1800s until a century later, when they were superseded by hard-plastic models. You should be able to identify them with relative ease.
Look for initial signs of age by glancing through the doll's clothes. Assuming they are original, they won't make use of Velcro or poppers. In addition, you will probably find traces of trapped dust and occasional signs of perishing or fading.
Establish whether the doll is porcelain by comparing the head to the fingers. Because the former is supposedly porcelain and the latter should be composition, the head ought to have a much finer finish. On most late 19th-century porcelain dolls, you should also be able to remove the wig and peer inside the doll's head. Glazed porcelain heads will be glossy on the outside but matt within. Another way-- not for everyone--of testing whether a head is porcelain is to touch it with your teeth. In comparison to other possible materials such as composition, wax or plastic it should feel hard and cold.
Check the back of the doll's head for a maker's name and mould number-- not all have them, but many do. By typing these into your computer's search engine, you should be able to find similar dolls listed on online stores and auction sites. The accompanying descriptions will probably include a rough date.
From 1870 onwards, you see porcelain dolls with wigs and inset glass eyes. Earlier in the century, the hair was moulded porcelain and the eyes were painted on.
Think twice before buying a porcelain doll with a cracked head, because the head is where the value of the doll lies.