Having many years ago had Brother printers that made it easy to print a
test or information page, it was good to finally find out how to do it
again. Of course, if you have the nice windows drivers, hidden in there
somewhere you can just click "Print Settings" and it will print out the
toner life, number of pages printed, etc etc. If you are using another
OS though, you won't always have that nice button.
Thankfully the trick is fairly simple. If your printer is on but sleeping (The large "Go" button, the blue light one is dimmed) then just press the button once to wake it up. Once awake, press the big blue button 3 times, slow enough that it can register each press, fast enough that you don't take all day doing it. You get the general idea. And out will come a "Printer Settings" page, complete with a "bar" showing remaining drum and toner life, total pages printed (even down to the pages printed for each paper size), and error history and what page it happened on. Apparently it even counts paper jams.
Now I have to warn, that it doesn't always seem to get things right. I'm sure I've had paper jams, yet it thinks I've had none. (Maybe I'm remembering an older printer, I won't complain). It also says I've only done about 1000 pages since getting the printer, now this I'm sure is wrong as I printed 100's of pages for uni last semester.
Regardless, it's probably a good idea to print one of these pages when ever you change the toner, so you can get a good idea of how many pages you are getting out of each toner!
NB: This may or may not work on other Brother printers. It should work for all the HL-2140 range (including the HL-2142 which is just the HL-2140 branded exclusively for office works so no one can price match).