If you are buying used brakes from someone who has no clue, ask if you can get good pictures. Then take a good long look at examples of long, medium, and short brakes on google image or whatever site you like. You should be able to get a rough idea what reach the brakes are by comparing them to known examples. From looking at bike stuff constantly, I can usually identify a bike's frame size within a cm just by looking at a picture.

Edit: To determine what reach brakes your bike requires you will need to measure as others have instructed. You can then approximate the reach of the used brake you are looking to buy by comparing it to known examples. If you don't want to risk buying the wrong brake, buy a new set with the reach your bike requires.