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

• The printer's lights will flash when one or more of the ink cartridges is low or expended. You may also see a message on your computer screen.
  
  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.
  Related references
  Replacing Ink Cartridges and Maintenance Boxes
  Printer Light Status