Updated 7. june 2010: added source code and binaries for KeyTest3AK with prechecked WM_KEYDOWN, WM_KEY_UP, WM_CHAR and option to save a log.[Download not found]
I just finished my KeyTest3AK. You can take alook at what keycodes produced by the keys on your handheld. Additionally there are options to test what happens, if you use GXOpenInput(), RegisterHotKeys(), UnregisterFunc1 and AllKeys().
After you started the app, simply press the keys to see the codes they produce.
Download source code and the executables [Download not found]
Here is MS answer on KeyTest : http://blogs.msdn.com/windowsmobile/archive/2009/05/14/twisted-pixels-4-a-button-mashers-guide-to-input.aspx. The MS blog has all the background on KeyInput but looks like a clone of keytest3ak