What advice would you give a customer looking to hire a provider in your area of work?
First off, it is imperative that you have reviewed the designer's portfolio of work. Although design is subjective, you have to ask yourself if they were able to address the project goals and engage the target audience. Don't just look for examples of your specific project, an overall understanding of design means they can solve your challenge as well. See if the designer has a set style, or if their work represents the needs of each individual project. Remember, you don't want the designer's stamp on your project, you want your brand's stamp on the project.
The second item is to set-up an meeting/conference call so you can get to know the people you will be interacting with on a regular basis. Solving design challenges should be done collaboratively, not in a silo. Remember, you know your company/brand better than anyone, however the designers are the communication experts so it is absolutely necessary that you work together. If after your meeting you feel a chemistry then it's fairly safe to assume that you'll have a positive working relationship.
Lastly, get an understanding of your project timeline and the availability of the prospective designer. You want to make sure they're going to give your project the time and energy necessary to correctly solve you challenge.