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.
  Note: If any of the cartridges installed in the product is broken, incompatible with the product model, or improperly installed, Epson Status Monitor will not display an accurate cartridge status.

  You may also need to replace a cartridge that is more than six months old, if your printouts do not look their best even after cleaning and aligning the print head.