I am not sure if you mean babysitters coming into your home, or outside centers and family home daycares. Either way, I'm going to say recommendations is the big one! For a babysitter, ask them for references from past families who they have babysat for or past employers who have knowledge of their work with children. While "licensing" programs for babysitters don't really teach too much of anything (i'm thinking of certification courses from the red cross or similar) I would want a babysitter with First Aid and CPR certs.
As for center or home-based daycares, I would ask them if they have testimonies from past or current families. Observe a day in their center. And also ask about their drop-in policy, and whether or not parents are allowed to stop in during the day. Some places limit this b/c too many visits can disturb the educational process, and also disrupt children from transitioning smoothly into the center, but if parents aren't allowed to drop-in at all I'd be a bit suspicious.