To quickly test how your form will be displayed when it is compiled and run, click the Test Form button in the GUI Builder toolbar. A dialog box is displayed with the components arranged as they would actually appear on your form.
When you click in a Testing Form dialog box, mouse events are delivered to the actual components and you can see the components "in action." Thus, for example, you can move sliders, type into text fields, and buttons look "pressed" when you click them, however, cross-component and event handling code is not executed.
To test your form without compiling and running it:
The IDE displays the form in the Preview window enabling you to test the behavior of your GUI.