The majority of cameos face right so if you find a left facing cameo, it is considered more unusual. A cameo's lines should be clear and well executed. The term high relief is used to describe cameos that are very deeply carved and have significant depth. If the cameo features a full figure, check to see if there is a significant detail in the feet and hands indicating excellent carving work.
Cameos were very popular in the Victorian era (which lasted from 1837 until 1901), through the early 1950s with many pieces being classified as mourning jewelry. A cameo habille means a dressed cameo so these have little stones, sometimes diamonds, embedded in places like the figure's hair or strung on a chain around the ladies neck.
The features of the cameos have changed over the years. There is a saying 'The Nose Knows' that's applicable here because we can date some cameos of women based on the shape of the nose. Long aquiline noses were the main stay until the 1940s and up turned noses were gradually phased in and replaced them. Cameos portraiting buxom women with thicker necklaces were the fashion through the Victorian era and first third of the 20th century, while the newer ones reflect more elongated, narrow necks on slimmer ladies. Cameos with flowers adorning a woman's dress were popular during the 1920s and through the 1950s. Not all cameos are simply busts of women. Male cameo heads were also carved mostly in the Victorian era. Scenic cameos such as ones with biblical scenes were a frequent Victorian theme and can feature dragons, gods and goddesses, chariots, etc. Many of these are highly prized and rare today. If you like scenic antique pieces, the most popular late 1800s scenic cameo was Rebecca at the well featuring typically a tree, a woman and a building with some variations.
Examine the cameo carefully for any tiny chips or excessive wear which results in blurred features. It is a good idea to bring a jeweler's loupe with you to inspect the piece. You will want to see if a cameo is hand carved or molded plastic since hand carved are more valuable and desirable. Check to see if the piece is cold to the touch and hold it up to the light. You should be able to see through the transparent areas of the hand carved shell. As you do this, look also for any signs of fissures or cracking. As cameos age, they are prone to this. Antique cameos are fragile and some owners gently rub olive oil on their pieces to keep them from becoming brittle.
You will also find antique madeos made out of different materials such as lava, gutta, percha, and angel skin coral shell as well as abalone, onyx, old plastics such as vulcanite and Bakelite and celluloid. There are lovely ones to fit most budgets and tastes but please be aware, collecting cameos can be habit forming!