When you leave PR. there is an immigration desk in the concourse before you get to the gates. They ask you if you are a US citizen, usually only if you look hispanic. This is because there are alot of illegal aliens that come over by boat from the Dominican Republic, and then try to fly to the mainland US. If they become suspicious, they might ask you for passport, ID ,ect. but there is no "official" passport control, since PR is a US territory.