Go into service mode, click Copier, click Miscellaneous or Misc, and you will see the drum life displayed for each drum. There's also an option to show bars on the normal display but these aren't very accurate and won't tell you anything beyond 100% yield (just show zero bar) Be very very careful and don't press anything you don't understand in service mode or you can destroy your machine.
If you haven't already, make sure you run a Full gradation adjustment - click the additional functions on the keypad, Adjustments - Full Graduation Adjustment. (don't mean this as an insult if you already know this, just don't know how long you've been using the machine for.)