If testing line, you don't use any knots, it is just wrapped around a fixed end, and stretched until it breaks by a moving gizmo with a dial gauge attached, it could be done kind of OK by wrapping one end around a smooth object, and doing the same on a spring scale, by adapting something that won't cut the line, then just pulling while reading the weight. A better method would be to measure the exact diameter with a micrometer!