A cameo is a gem, hardstone, shell or some other substance that had been carved to show a figure or scene, generally (but not always) with contrasting colours for the background and foreground. They were first worn in ancient Rome. In the Renaissance, antique examples were remounted in elaborate new settings of gold and enamel. In the 17th and 18th Century, the art of the cameo was revived by connoisseur princes. In the 19th Century, mass-produced examples were purchased as tourist pieces, and they remained popular until the 1940s.
Examine the setting. This is perhaps the easiest way of dating a cameo. A good example should be set in gold. On brooches, look at the pin. If it has a safety catch, then it is a relatively modern example. Also consider the style of the mount. From the 1820s through to modern times, most cameos were given a classical setting (because they were bought by tourists in Italy), often with a rope-like design around the outer edge. Before then, the mounts frequently included enamel and jewels.
Look at the cameo itself. What is it made of? Very early cameos were carved from semi-precious gemstones, then, from the Medieval period through to the 18th Century, hardstones such as onyx were used. In the 19th Century, a type of pink and white agate called sardonyx was popular, and it was a desire to imitate sardonyx which led to the use of conch shells towards the latter quarter of the century. You can easily distinguish a shell cameo because it will have a concave back.
Inspect the carving. How crisp and subtle are the details on the drapery and hair? Some cameos would have been hastily knocked out by journeymen, while others were the work of artists. On shell cameos, heads in profile are generally later in date than full figures.
Consider the face on the cameo. These tend to change subtly over time. If you see a nose or a hairstyle which reminds you of a 1950s' fashion catalogue, then it's probably a 1950s' cameo.
The most important question when judging a cameo isn't how old it is but how well it is carved. Superbly executed examples in gold mounts will always command a premium with collectors.
Watch out for fake resin cameos. Look for dollops of glue on the rear of the mount.