To preview or debug a form, you can click on the appropriate button in the Question Editor:
The button on the left is Preview and next to it is Preview with Debug.
Preview of a form allow you to do everything like a Published form, except the Submit button is not available.
Preview with Debug is same as preview, except there is a debug dialog box showing you the change of Conditions on-the-fly:
In addition, when you mouse over an option with condition setting, there will be a balloon floating and shows the condition settings associate with the option.
For more information on Condition Engine, please visit this section.