On HF6 there exists a menu option under XFCE: bring up the menu, choose settings, and select Pandora Input Tester. It also exists in Minimenu under the System tab. If you're on HF5 or earlier you can open a terminal and type "op_test_inputs" on the command line for the same thing.
What you'll see is a representation of the keyboard. Don't worry about the nubs, they need to be specially setup before running this app to do anything, but we're ignoring that setup for now.
An unpressed key will be grey, a pressed key shows as green. In HF6 an additional feature was added: a key that was pressed but has been released will show as yellow instead of grey. You can use this program to determine if your key is not responding properly.