If you have pre-paid your gratuities, it usually covers your room steward, waiter, assistant waiter and the head waiter. If you order drinks from the bar, a service charge is automatically added into the total before you sign for them, so don't add an additional tip onto that.
Other service charges you may want to add on are for any spa services you decide to have.
At any time during your cruise, you can go to the Purser's Desk and ask them for a list of charges that have been made so far to your room. They'll print it up and give it to you.