How can you tell if someone is qualified as a plumber?

If they are a licenced plumber,they will have a card issued by the state.They may be knowledgeable in this area without a card, however, depending on the extent of the work you are asking them to do, make sure they are knowledgeable in that area by asking them how they propose to do a particular task,and if the answer doesn't sound right, get someone else.