Hi there, I'm just in the process of setting up a piggie boarding service, so all this is pretty fresh in my mind at the moment! LOL!
So, first off I'd go with my gut feeling about the place and the people........are there any large or yappy dogs roaming around, are there any small children, are the people smokers (assuming the pigs will be indoors), does the place seem calm and organised.
Then I'd want to know practical things like size of cage / indoor or outdoor living / type of bedding / do the piggies get exercise / what feed, hay and veggies do they give / how often are the cages cleaned out / are the cages thoroughly cleaned between residents.
I'd also want to know whether they had insurance and what that covered, what experience they have with guinea pigs (most of the small animal boarders I've looked at seem to have mostly rabbit experience), what would their procedure be if there was an emergency situation with your piggie, do they have a relationship with a cavy savvy vet? Stuff like that.
Hope that helps! Let us know how you get on. Tracey x