Digital BW, The Print

I found a trick that works in trying to figure out what color inks a 
printer is using for BW output is to peek inside the printer while 
it's printing and look at the output as the head is laying down the 
ink. It usually takes several passes of the head to build up full 
density so after the first couple of passes you can see what's going 
on before the ink buildup obscures it.

Another technique is to abort the print job half way thru the job 
then look at the ink buildup zone with a loupe.