Software is now available for you to check on the pesky dog walker. It has a bar code that the dog walker scans, and a GPS device for you to track the dog walker's walk.
This is a perfect example of how technology does not solve all problems.
A determined cheat will find a good, convincing way around this solution.
More than that, if your dog hasn't been walked all day long, don't you think you'd be able to tell? They would have to go to the bathroom, so to speak, and would be restless.
As a dog walker, the answer to this worry of clients, and they all think you are secretly not doing your job, is to leave little notes about what happened during the dog's walk. Such as: we met and played a little bit with a brown puppy down the street. We went to the park and ran up and down. Things like that.
The important thing is to build trust with your clients. More than that, recognize that dog walking is serious manual labor and there are no shortcuts. Someone approached me on the street asking that once. I suggested he find another line of work.