How can I know whether my laserjet is using color toner to print black?

I've got an HP M176n all-in-one color laserjet printer. For quite a while now it's been displaying black toner low messages but I've been ignoring them, preferring only to replace the toner cartridge when I feel the black is faded enough. However, the black still seems fine and I'm starting to wonder whether the printer is compensating for the lack of black toner by combining the colors to create black instead. I generally print black and white stuff the vast majority of the time. Might the printer use the color toner instead if black is low, and how can I tell whether it's doing it?

Question by Jez on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.