The UI industry is full of vague. We are not.
GeneralWhat do you do long term for my UI? How well is it tested? How long have you been working on UIs? What is the difference between UI & UX? Who will be doing the work? Jnr or Snr?
SpecificCan you show some examples of your work? What what your exact involvement? Do you write your own code or buy/paste others?
Mobile AppsHave you worked on iPhone apps? Have you worked on Android apps? What is the difference? How many & for how long? Do you know Apple’s Human Interface Guidelines?
SitesWhat is the highest converting page you have ever made? What was it for? What is the biggest site you have built? What is the busiest site you have built? What is the longest time on site you have achieved, what was it for? What frameworks do you use?
CopyWho will write the copy? What voice will it be in? Can you handle multiple languages?
Feel free to ask us any of these questions or any others you may have.