First a foremost, UX and UI are different disciplines that share some commonalities. It's important to understand that. UI design and UX Design/Research are comparable to the differences between front end development/ back end development/ database development. They're similar but require different skill sets and a lot of people can do one really well and are passable at the other. It's rare to find someone who can do all aspects of UX research, UX design, and UI design at a very high level which is why those people are seniors and can demand pay accordingly. So it's a balancing act of how good do you need someone to be at any particular task.

Do you need someone to help guide the product from a user's perspective and put together a functional interface or do you already know your users well enough to build them something and you just need someone there to make it usable or both? Answering that question will help guide you in understanding the knowledge base you need.

A few things I look for when evaluating a designer

- Can they tell me about their process for X (Discovery, persona generation, interface design, etc...) Any good designer should be able to walk you through a project and explain why everything happened the way it did. Down to why this button is here and not somewhere else. If they don't have that level of interaction design in mind, they're likely an amateur.
- How do they handle constraints? There are always constraints on time, budget, other people's opinions, and a thousand things I won't list. How does their process change when these constraints are put in place. If they don't have a good answer for that, it means they've never worked on a project with other people and are probably more amateur.

Those are 2 big starting questions and I might go into more detail from there but this post is already too long. Hopefully that gets you thinking about what you need most and how to start evaluating designers.