How would you look for a contractor for the design and design of interfaces?

Now the Russian digital market is filled with full-cycle web-studios developing turnkey projects, various SEO-, SMM- and advertising agencies. The competition in these areas is quite high, a huge number of various ratings appeared, and even aggregators of ratings began to appear. That is, in principle, there are no problems with finding a contractor, if at the exit you need to get a ready-made site with promotion and support. However, there is a category of customers who already have their own development department, and they are also engaged in promotion independently. All they need is to find an outsourcer who could design and draw the interfaces efficiently and on time. Most often in such a situation it is not a simple information site, but a complex and unique web service, which brings the main income to the customer. No specialized ratings were found on this topic, and those who are familiar with the necessary recommendations are not always available. Those wishing to take part in the survey please under the cat.

For illustration, I will cite as an example a fictional case. Imagine that you need to find a contractor to create an interface for a "self-made" CRM car dealership. You already have developers. Most likely, you want to find a contractor not only with extensive experience in design and beautiful design work, but also that this person or company already has several completed CRM projects for other car dealerships. What would you do?