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

• When the printer's ink out light flashes, an ink cartridge is low on ink. This is a good time to make sure you have a new cartridge handy.
  
  When an ink cartridge is in the printer longer than six months and the print quality declines the ink cartridge should be replaced.
  
  When the ink out light stays on, the cartridge is empty and needs to be replaced. The printer will stop printing.
  
  You can click on the EPSON Status Monitor to check which cartridge is low or empty. To check your ink supply, do one of the following:
  
  Windows:
  Double-click the printer icon on your taskbar (in the lower right corner of your screen). You see a window showing ink levels. You can also double-click the Ink Monitor icon on the taskbar. The window that appears lets you check ink levels and also order cartridges online. Just click the Buy Now button to launch your web browser.
  
  Macintosh:
  OS 8.6 to 9.x:
  Choose File > Print. Click the ink icon.
  
  OS X:
  1. Choose Applications.
  2. Double-click on EPSON Printer Utility.
  3. Select SP 900 in the printer list.
  4. Then click on EPSON Status Monitor.