Where would I find reviews on Dakota walleye fishing guides?

The internet is your best bet for Dakota walleye fishing guides. Avoid reading testimonials that a site has in place. They are only going to reflect the good information that has been reported. To get the pros and the cons though you want to go to the source. There are plenty of consumers out there that will share their own experiences with you. They may do so to share information or to warn others about who NOT to hire.
If you have a particular fishing guide in Dakota to consider, then you can narrow down the search by looking at them by name. Find out what others have to say about them before you hire them yourself.
You can also find such reviews out there on various forums. They can be harder to locate though because you have to create an account and then search for the information. Still, they can help you to get the information you seek to make up your mind about who to hire.