I think keyboard designs are similar enough to have a standardised test. I suggest the following:
1. Q-W-A-S
2. Q-W-E-A-S-D
3. Q-W-E-R-A-S-D-F
(and so on...)
When a failure is encountered (i.e. not all the keys register), try to reduce the number of keys to find the minimum at which the failure occurs.
Most non n-key capable keyboards would fail on 1. My guess is that keyboards limited by the USB protocol would pass 1 and 2 but fail on 3 (but I don't have one with me to test right now).
We can transpose these tests to different locations on the keyboard, eg a few keys to the right, or up or down a row. But I suspect that wouldn't make a difference, unless it's an unusual keyboard design. If we encounter those we can always add more tests to the list. 
I am thoroughly confused but here's some tests anyway. The tests below are done on the Unicomp Customizer 101 (manufactured Sept. 2008). PS2 cable connected to my PC via PS2-USB adapter (purchased from clickykeyboards). Windows Vista.
qwas: sometimes 0, 1 or 2 keys register for a given trial.
qwa: sometimes 0, 1, or 2 keys register for a given trial.
asz: all 3 keys register every trial
sdx: all 3 keys register every trial
3edc space: all 5 keys register every trial
qwer: all 4 keys register every trial
Referring to the first two tests where I get different results per trial...Perhaps I'm a klutz who can't press all keys at same time consistently if the keys are not on the same row or column.
What I really wanted to say in this thread is that that I play COD4 all the time with my Unicomp Model M and the 139401 (1988). I score high and both keyboards serve me well. They never let me down. Every key or move executes beautifully. Furthermore, I'd imagine when Wolfenstein or Quake were paving the way for FPS in the early 90s..people were using Model Ms. I'm not sure if anyone complained back then? I'm certainly not complaining right now.