I have used 5 dogwalkers to date. This is over the course of 8 years. They are not all the same. I first got names from the local vet surgery (and did this also when I moved to the area I live in now). Otherwise, as you get to know other dogowners when you are out walking, they may have some good recommendations.
The first two dogwalkers job shared. One was lovely and my dog loved her. The other one was a bit pushy (nothing I could put my finger on but I didn't entirely like her). With the second one, I sometimes came home from work and was unconvinced that my dog had actually been taken out at all. I started leaving the lead in certain patterns to check, etc. One day I came home and knew she hadn't been out since I left in the morning. I took her out of the back garden gate (opens on to a park) and walked her to the other side of the park. I looked back and could see the dogwalker just coming out of my garden with her daughter and looking around the park. She later said that she had just come back for the second time so her daughter could see the dog and looked a bit embarrassed. When I moved, I tried a dogwalker I'd found online. We had to meet her somewhere for 2 minutes so she could meet my dog and she was about 20-ish and not terribly friendly. I was desperate as work started in 2 days so I went with this. It never worked. First of all, she came to collect my dog and the dog refused to go out with her as she thought she was being dognapped. It took her 20 minutes to get the dog into the van. Then she had to walk my dog alone as she was too slow (old dog)and couldn't keep up with the group. Then I got a call at work to say the dog had bitten her and she was taking her home in a muzzle and rushing to the doctors and would never walk her again. I was very shocked (dog never bit people) and sent flowers, apologised etc. Eventually the dogwalker admitted she (large girl) had overbalanced and fallen on top of my dog (and her dog) and a dog fight kicked off and she tried to stop it and my dog accidentally bit her (my sister asked me if the dog needed a course of antibiotics!). The vet receptionist gave me details of another dogwalker who came round to the flat and met my dog, who really liked her (so did I) - a middle-aged lady, smiley, straightforward and calm - just the job. We never looked back. I once saw the dogwalker who had expelled us walking a very unhappy dog in the forest and felt very sorry for it.
So, meet your dogwalker AT HOME and go for a walk with him/her if you get the chance. If your dog doesn't seem to like her/him, take note and look for another. If you come home and your dog is a bit stressed or not their usual self, could be they don't like the dogwalker. Though I think, if you feel you can trust the walker and you take to them, the dog probably will too.