If you're trying to determine whether you are allowed to enter if you live in Puerto Rico, start with the sweepstakes' rules. Some US sweepstakes say that entry is open to Puerto Ricans, and others specifically say that you can't enter if you live in Puerto Rico. Some, however, don't mention Puerto Rico by name. So what next?
Some sweepstakes' rules will mention US territories, either to allow entry or prohibit it. Puerto Rico would count as a territory.
Some sweepstakes say that you can enter if you are "a resident of the 50 United States and DC." Since Puerto Rico isn't a state (and isn't DC), then you are probably not allowed to enter.
If the sweepstakes' rules simply say that you must be a US resident to enter, then it's ambiguous whether entry is open to Puerto Ricans. In this case, you can check whether there is a drop-down menu for the state. If Puerto Rico is not included on the list, it's likely that you are not eligible to enter.
If none of those tips tell you whether you're eligible to enter, you have two choices. You can contact the sweepstakes' sponsor to ask for clarification, or you can enter, knowing that you will be disqualified if entry from Puerto Rico is not allowed.
By the way, it's possible that more companies may start allowing Puerto Rico residents to enter their sweepstakes. In November of 2009, some changes in Puerto Rican sweepstakes law made it easier for companies to include Puerto Rico in their sweepstakes.