How can I unit test a GUI?

Designs like MVP and MVC typically try to abstract as much logic out of the actual GUI as possible. One very popular article about this is "The Humble Dialog Box" by Michael Feathers. Personally I've had mixed experiences with trying to move logic out of the UI - sometimes it's worked very well, and at other times it's been more trouble than it's worth. It's somewhat outside my area of expertise though.

Of course, the answer is to use MVC and move as much logic out of the GUI as possible.

That being said, I heard from a coworker a long time ago that when SGI was porting OpenGL to new hardware, they had a bunch of unit tests that would draw a set of primatives to the screen then compute an MD5 sum of the frame buffer. This value could then be compared to known good hash values to quickly determine if the API is per pixel accurate.

You can try UISpec4J is an Open Source functional and/or unit testing library for Swing-based Java applications...

There is Selenium RC, which will automate testing a web based UI. It will record actions and replay them. You'll still need to walk through the interactions with your UI, so this will not help with coverage, but it can be used for automated builds.