How do I know when an ink cartridge needs to be replaced?

• The printer's LCD screen will display a message when one or more of the ink cartridges is low or expended. You may also see a message on your computer screen. See Replace Ink Cartridges.
  
  Note:
  When an installed ink cartridge is more than six months old, you may need to replace it if your printouts do not look their best even after you clean and align the print head.