>>8
No, they actually have large machines with carefully measured stones that each weigh 6.35029318 kilograms.
What you do is take a stone from the machine and place it in the holder that you stand in front of. You keep adding them until the bar that looks much like a ruler starts leaning to the right of the little pointer thing in the middle, meaning it's now heavier than you. Then you look closely at the marked points and read to see how much of a stone the difference is. Or you can just round up or down. Usually women round down, of course.