User Testing

User testing is the best way to discover usability problems in your application.  If your application is a big investment for you, then you will want to ensure it will work well with actual users.  Bringing in representative users and having them run typical tasks with your application while an observer takes notes will help pinpoint not only the problem areas but will also validate the good design features and functionality in your software.  Although great for fully functional products, this method is also encouraged for prototypes in the early stages of development.