Most studios today in Italy have at least 2 master carvers on staff. There are some very fine plastic cameos out, so beware. Since a shell cameo is a shell the back is curved. Also you can see little carvng marks. Also you can get familar with the coloration.
Most plastic cameos have a different coloration than shell.
You can always do the hot needle test. If it melts, it's plastic. Finally do not get overwhemed when shopping. I find people usually buy the first cameo they are drawn to. Cameos are a great investment.