M.A.
Always do a background check on the person yourself, even if an agency did one. Try to get some insight on the person through the people whom she has worked for before. It may be hard though because me for example I have been a sitter/nanny for 15 years but over the years I have lost contact with the moms. So don't dismiss a sitter because she no longer has references, but do try to get some. Make sure the person is cpr certified. If the person is young try to get a feel for her home life. When your child is left in someone elses care please ask neighbors to keep their ears and eyes open for anything you disapprove of.