Sight measuring techniques are the cornerstone of observational drawing and painting, yet it can be difficult to find good information on how to go about it. Here are a couple of diagrams that illustrate this basic skill, I hope they will answer some questions and help people develop good observational practices:
Edit: I realized I didn't really describe the "method" very well first time around...
A) Hold the measuring tool (a thin skewer works best) at arm's length with elbow gently locked (this maintains the distance to your eye).
B) Close one eye and use your thumb against the tool to gauge the particular width/height/distance you are measuring.
C) Simply move the tool to check or gauge how many units the next width, height or distance is.
D) Transfer the relative information to your drawing - it is not a direct transfer of the distance by the way - but a ratio based on the size of your drawing.
(Note: When sighting a foreshortened angle or one in perspective, do not let the tool "tip" in the direction of the angle - keep it flat to the picture plane at all times. Sight measuring works much better when working with your drawing surface vertical at an easel or mule and placed just to the left or right of the subject. Also do not confuse sight measuring with "sight-size" approach - which just takes this method to a much higher level.)
(Note 2: Most artists I know use sight measuring in one way or another - the more experienced just tend to do it without the need to measure with anything but their eye. I'm not sure I used any "measuring" except by eye when I did this 30 minute sketch - it just turned out to be a good pose to show how to check all the information.)
Subject: 30 minutes, charcoal, 18x24
Define a "Unit of Measure" (usually the head works well but not always)
Use that unit to gauge relative distances, heights, widths, etc.
Also compare negative spaces, check angles, find landmarks, check overlaps, etc.