You will need some basic information, to make sure you are happy for your child to live in the school. This will include checking out the living accommodation and ensuring you have confidence in the house staff; the quality and suitability of the food; arrangements for when children are sick; ease with which pupils can stay in contact with family and friends (phones, email etc); the number of exeats or flexible boarding arrangements and whether these fit in with your family circumstances.
But you will need to make more subtle enquiries into the nature and quality of the boarding population and experience on offer. Some schools almost empty out at weekends while others have few exeats and few weekly boarders. If your child cannot go home at the weekends (if you live abroad, for instance), make sure there will be plenty of other boarders around at weekends. What activities are available out of school hours, especially at weekends, and how much of the day is scheduled and organised? Is the school geared to boarders or is it really a day school with a few boarders? Are pupils allowed into the local town, and if so are they accompanied or unaccompanied? Are the arrangements appropriate to the different ages?