You can preview and render data in your active render channels in several ways:
Click Render > Render Current Pass, Render Selected Passes, or Render All Passes in the render manager. The channel images are output automatically.
Right-click on a row in the Render Channels Output table (on the Output tab of a pass' render options) and choose Run Flipbook to launch that render channel in the standalone Flipbook player.
You can display the output for each render channel in a render region, see Displaying a Render Channel in the Render Region.
Choose Render > Preview from the Render toolbar, see Previewinga Single Frame.
When working in the render manager or the render tree, shaderballs give you a quick preview of a material or shader. They update automatically as you change settings, see Using Shaderballs.