If you want to get a rough estimate lay a piece of masking tape along the length of the ram rod and put a vertical line on it. Put your jag on and start a patch down the bore. Line up your line with a point on the octagon barrel and mark the rod at the muzzle. Push the rod down slowly allowing it to twist as it follows the rifling. When the line comes around (or half way around depending on barrel length) mark it again. Measure the distance between the marks and you got your rate of twist. Its not exact as it may skip but at least it will give you a rough idea.