Mac users previously had the ability to preview their AE comps over Firewire or a video card. Now, with the release of AE 6.5, PC users can too! Plus Adobe has enhanced the ability of the preview all around. It is possible to preview the contents of a Layer, Footage, or Composition window on an external video monitor.
You can set Video Preview preferences by choosing After Effects>Preferences>Video Preview (Edit>Preferences>Video Preview). Choose one or more of the following interaction types.
Previews: Both RAM preview and standard (spacebar) preview load on the external monitor. Choose Mirror on Computer Monitor to if you want to see the preview simultaneously on the computer screen and the external monitor, however mirroring reduces performance.
Interactions: Interactive previews such as scrubbing and dragging in the Composition window on the external monitor. Interactive previews are always mirrored.
Renders: Each frame is shown the external monitor during a render. All renders are also mirrored.
You can toggle video output by pressing forward slash "/" on the numeric keypad. This is a great way to turn this handy feature on and off.