Binocular style range finder
Disclaimer: I'm guessing here, based on how the focus works on my camera.
Look through the viewfinder. There should be a horizontal line in the center. There should also be a focus wheel or knob somewhere. Look at something (preferably with distinct vertical lines, like a building or trees) and turn the wheel back and forth. When it's focused, vertical lines on the object you're looking at will be aligned above and below the horizontal line. Read the distance off the scale.