Use this page to test your keyboards N-Key Rollover
Using This Demonstration
Click on the keyboard shown below. It will then show what keys are being pressed on your keyboard (or to be more specific, the ones that are being passed to your web browser). You can use this to see if there are certain key combinations that fail to work correctly.
If you prefer a more fool proof method of testing your N-Key Rollover, AquaKeyTest is a Windows based program that installs locally and listens to the output your keyboard is actually sending to the computer. This program can also be used to test every key on your keyboard, and is not faked out by scripts or any hotkey shortcuts or macros that may be on the keyboard.
How To Hunt For Ghosts
Ghosts can be somewhat elusive. Just because you find some combination of 6 keys that reports correctly does not necessarily mean that every combination of 6 works. In fact, the most straightforward keyboard design has thousands of 3-key combinations that do not work! That is a lot, but it is still less than 5% of the total number of 3-key combinations.
A quick way to find problematic combinations is to press and hold ASDW (the most popular keys for moving around in games), and then while holding those down, press every other key in turn. If you find a key that does not work, try releasing some of the ASDW keys. Odds are high that you will find a 3-key combination that fails.