How can you tell if it's an antique cameo? That's a good question! The picture on the front will usually give you a pretty good idea. If her hair is cropped into a shorter hairstyle (not a long unswept hairstyle, as this can indicate antique cameo) and he/r nose is pointed, not straight roman nose, forget it, it's not an antique cameo.
Market knowledge: the cameo jewelry market is climbing and there is a definite trend toward younger buyers with an eye for quality.
A cameo ring usually sells best and well carved antique cameo earrings are always in demand. When purchasing cameo pendants or brooches, have in mind that quality cameos were usually mounted in quality metals. (There are exceptions)
CONDITION- MOST IMPORTANT - Inspect the antique cameo jewelry under strong light front and back, for any stress lines, fractures, flee bits, preferably with a jewelers loupe or strong magnifying glass. Any lines or fractures will significantly affect the cameo's value.