Hello Harry,
I totally understand your concern, I was the same when I had to go away for a week and decided to leave my babies with a pet sitter. But it turned out fine, they absolutely love their pet sitter and she even became a friend of mine. I think the best thing you can do first is meet the pet sitter outside with your dog to see how he/she interacts with your pet. That will already give you a hint if you can trust this person or not. Then ask him/her questions about previous experiences, what's included in the services and why does she/he enjoys being a pet sitter for example.
I'm sure you will find someone great to look after your pets. Good luck :)
