Result Preview
Of course, you don’t need to work blindly. You can have a preview image that shows you real-time updates of what you’re doing in the Node Editor. To enable a preview, you need to create a Viewer node. The Viewer node is inside the Output nodes group. Add it to your workspace and connect the output of the node you want to preview to the input of the Viewer node. The Viewer node will display a preview of the result of all the nodes in the node tree that affect the node you connected to the Viewer node.
An even faster way to enable a preview is to press Shift + Ctrl + LMB click the node you want to preview. This will automatically create a Viewer node and connect it to that specific node. Pressing Shift + Ctrl + LMB click in any other node will quickly connect that node to the Viewer, so you can see the effect of the node you want to preview really fast.
Once you have a viewer node in your node tree, you have two different options for previewing your work in the compositor.
Backdrop: In the Node Editor’s header, enable the Backdrop option and the preview will be shown right behind your node tree, in the background of the Node Editor workspace. Press Alt + MMB to pan it, V to zoom out, and Alt + V to zoom in.
UV/Image Editor: Although the background preview lets you see everything in the same window, it can be very distracting at times and the nodes on top of the image can prevent you from seeing what’s going on (especially if you have a complex node tree). In these cases, or if you just want to see the result on a secondary monitor, there is an easy way to do it: open the UV/Image Editor and, from the images list on the header, select the Viewer output. This way, you’ll see the Viewer node preview as if it were an image in the UV/Image Editor.
Note
Don’t forget that in order to see what you’re doing in the compositor, you have to render your scene first (unless you’re working with images already rendered or adjusting videos instead of a 3D scene render).