Miriam Ellis recently wrote a blog post about this on Moz, which I would definitely recommend checking out:
https://moz.com/blog/local-seo-client-discovery-questionnaire
Within it, she provides quite an awesome template or "Client Discovery Questionnaire" of questions that she recommends sending to new clients to get a better understanding of the client's history including other consultants that they have worked or are working with. This is available to download as a Google Doc too! It's a great resource.