Hmm... Well, with the one we ended up going with, I talked to her on the phone and asked to meet with her in person; she brought a new-customer information packet with a reference list, a local vet recommending her, her experience, etc; copies of her contract for both of us; and also a really thorough health history for us to fill out about all of our pets -- all of which seemed professional enough that I felt pretty comfortable with her.... This was the first time we'd used a petsitter so I didn't know what to ask, really. I just checked out that she had experience with giving medications and with the kinds of pets that we have. Other than that, I liked that she made sure to meet and interact with each of the animals, so they'd know her a bit. She'd also been in business for quite a while so I felt that she would be responsible, since she had an established business to protect...
I did find out later, much to my surprise, that she brings her three-year-old with her and lets her run around the house. I thought that was unprofessional, since jeez, all kinds of stuff can happen with a toddler running around your house. I'd have wanted to move my breakable stuff at least, besides which I don't really trust little kids with small animals if I'm not there to keep an eye... So I think now, after having that experience, I'd make sure that the contract states that the petsitter is the only person I'm giving permission to be in the house, not them and whoever they feel like bringing with them...
Pricewise, one of the two I looked into charged something like $10 per visit, and the other $10/day, for one visit in the morning and one in the evening. Both charged a little extra for over two pets; and a little for extra things like giving meds.