When conducting the interview, ask the babysitter questions about hypothetical situation, and listen closely to their answers. Another important thing to consider during the interview is that you should not bring your child to it. It could distract you from the babysitter since you don't want to miss any details. Bring your child to meet the babysitter at some other time if you're still sure you want that one to take care of your child. It is good to see how the babysitter interacts with your child so try to set up a time when you can see the babysitter in action before you start the actual sessions.
Age is also part of the investigation.
If the babysitting is at a place other then your own look at the other children the babysitter has. If they seem dirty, unkempt, and the babysitter doesn't answer then nicely, chances are you don't want your child to be there. The place itself should be clean and well kept, and safe for your child. Watch for small parts, knick-knacks and other little objects that could pose a possible hazard to your child. Once you know you like a babysitter, and trust that they would be able to do their job responsibly and thoroughly, set up a probation baby sitting appointment.