UI and UX design are usually sold together and are not the same thing. UX is the shape of the product — what it does, in what order, and what a user is trying to achieve. UI is the surface: the controls, the visual language, the details of interaction. A product can be beautifully rendered and still ask people to do something illogical.
The most informative question for a provider here is what research they do before drawing anything, and what they do when it contradicts the brief. Teams that go straight to high-fidelity screens produce work that looks decisive and tests badly. Ask to see a project where the research changed the plan.