How do I know that they actually provide the service they are supposed to?
I've heard many stories of people who hired a dog walker or someone to do a check in visit only to find out that the dog walker did not in fact spend the proper amount of time with the dog.
If you are concerned, open up a dialog with the sitter, and then if you're still unsure, reach out to rover. Consider investing in an inexpensive security camera for your front door or the like. They have dropped in price and you can keep aware of movement of people coming and going in your home.
If you book the sitter through Rover they have to use an app that records when they are there.