You need to find someone you trust, if you don’t get along with the designer then step away. If the designer shows you samples of their work and you are not 100% convinced step away. You need to be able to give them a long wish list of requirements you want to have and wishes to the functionality and feel and look. The designer needs to be able to put at least 98% of your list I to the design. And I believe the education of the designer matters a lot as well.