After many uses, you will start to notice streaks and lines in the faxes from your machine. This is a tell tale sign that you are running low on fax toner. Too many people replace the fax toner right when this begins to happen. What they don't know is that there is typically 10-15% of the fax toner left in the cartridge that has simply been shifted away from the dispensing area.
When you start to see lines in your faxes, do the following. Take your fax toner cartridge out of the machine. Gently, give the cartridge a little shake. You can even use the bottom of your hand to gently bump the edges of the cartridge. This will loosen everything up. Then, place the fax toner cartridge back in the machine. Run a test page to get the last spots and lines out. You should have many more faxes from that toner cartridge before you need to replace it.