They take attendance at the muster drills. On the older ships, they call out each cabin number and you say, "Here" and mark you as there. On the newer ships like Oasis and Allure, your key card is scanned when you enter the room where you're assigned, so they know that way. Sometimes, stewards find out who is not at the drill and will go and knock on doors to see if anyone is in the room.
If you are not at the drill, depending on the cruise line, they request (insist?) that you attend a make-up muster drill with others who were absent. Obviously, it's very important that everyone attend a muster drill in case of emergency.
There are no cameras in the room.