I've had 3 different dog walkers and all have been great.
Do your research and look at reviews
Go with your gut feelings
Each time, they have come round and had a chat about my dog, his behaviour, on or off lead, etc
See how she reacts with the dog.
Ask about timing - if its an hour walk, is it an hour from door to door, or an hour of actual walking plus transport time?
How many dogs does she walk (I would not want anymore than 5 dogs personally)?
On or off lead?
Does she send photos/updates?
Key security -where is key stored
Van safety - are they ever left in van etc
Sometimes they ask you to go on a walk with the dog, but I have never done that.
My dog loves his walks, I was working from home once when the dog walker came, and he went crazy when he saw her van and didn't look back!
When she leaves, she checks the water bowl and gives him a chew from the treat box.
If he has got wet, she knows where the dog towels are kept.