How to tell is rover-sitter really comes, stays long enough, etc?

The Rover Card provides that information and is used for drop-ins and walks. I am going to provide a link to a detailed explanation of how Rover Cards work but it is from the sitter's perspective on how to use it. I could not find anything in the owner section of Help that addresses your concern. Here it is:

https://support.rover.com/hc/en-us/ar...

Be sure to discuss with the sitter that you definitely want to receive the Rover Card info and discuss what you want the sitter to do on the visit. Communication is always important to be sure your expectations are met.