Histograms

This is an example of a histogram.  Getting into the habit of checking your histogram as you shoot will help you see when you need to do a shot over again.  It is usually good to have some pixels all the way to both ends, and to have a majority of pixels somewhere in the middle.  But this is art, so whatever effect you're going for may look different from what I've just described.

High key images (mostly light values) will have the highest spikes toward the white end of the graph.  Low key (mostly dark values) will have the biggest spikes toward the black end.