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

• When the printer's red Error light flashes, an ink cartridge is low on ink. This is a good time to make sure you have a new cartridge handy. When the Error light stays on, the cartridge is empty and will need to be replaced. The Error light also flashes when there is a paper jam, so make sure you check your paper before changing the ink cartridge.
  
  To check your ink supply, do one of the following:
  Windows: You can either double-click the printer icon on your taskbar or double-click the ink monitor icon to display the ink levels remaining.
  
  Macintosh 9.x or earlier: Open the File menu and choose Print. Click the Ink icon. You'll see a window showing the ink remaining in each cartridge.
  
  Macintosh OS X: Double-click the Applications folder on your hard drive. Double-click EPSON Printer Utility, select the C42 Series printer in the Printer List window and click OK. Then click the EPSON StatusMonitor button. You'll see a window showing the ink remaining in each cartridge.