Ensure that you have visited and are happy with the size of accomodation and run faciliites. Ensure that security of both hutch and run from predators is adequate.
Check that they do not allow rabbits to exercise on grass as transmission of EC is possible due to inability to decontaminate grass. Find out how they decontaminate runs and hutches between boarding rabbits.
Ensure that you are happy with the person who is looking after your rabbits and their knowledge of rabbits care.
Only use the establishment if all rabbits who are boarded are fully vacinated.
Check that you are happy with the vet they use if problems occur.
Ensure that dogs are not allowed near the boarding quarters and that animals including ferrets are not boarded on the same premises.
Ensure that if your rabbits are indoor animals that suitable accomodation / heating is available.
Ensure that the owner will maintain a similiar routine for your pets as you do.
I am sure that there are plenty of other things I would check, these are some I can quickly recall. Most importantly if you are not happy or unsure look elsewhere.