If you're looking to test that a specific key is not down then I would do as stated above.
If you're looking to test that no keys are being pressed then that's not quite as straight-forward. This is all I could think of - I hope it works for you.
<img src="https://dl.dropboxusercontent.com/u/184657779/C2_demos/keynotdowntestusingkeycode.png" border="0" />