There is a couple of ways. If your C1 is on a network, open your internet browser and type in the IP address of the C1 you will enter the Remote User Interface. On the main screen you should see the toner levels. The RUI is a great feature Canon has for users to check the machine without having to go to the screen. Also you can check by hitting the counter check button and then I believe on the screen you should see consumables.
Let me know if you have any other questions
