Further information on Phaser 560 consumables - this time from Xerox Europe:
Fuser Roll: Each one has a fuse installed - the printer senses a new unit by the fact that the fuse is intact. Replace the fuse and the printer thinks it's a new fuser roll and resets its counter.
Other consumables: Life expectancy measured by average coverage per print - at an average of 5% coverage, life expectancy is 15,000 pages. There are no fusable links in these components.
Toner cartridges - a photo sensor is used to gauge whether the cartridge is empty or not.
I can't guarantee it, but it seems likely that other models use similar sensing mechanisms