Ask for a tour of the facility and make sure you get a chance to look at the kennels, vet facilities, spa, therapy areas, exercise areas as well as the general facilities lay out. If it houses several types of pets, are they kept separate and how is each area secured?
Some pet centers will not take particular breeds of dog or pets that are ill or have a history of aggression or running away. This is usually due to liability issues as well as safety of staff, visitors, other pets as well as your pet.
Find out about staff turn over at the pet center. If the staff is fairly consistent you can be assured that your pet will get to know the staff and handlers, making being left at the pet center or just visiting for grooming and vet checks more personal and less anxiety causing for the pet.
