Doesn't the barrel have the twist printed/stamped on it?
If not, Use your ramrod with a tight patch....Put a mark at 12 o'clock on the rod as you push the patch in. When/if you have one complete revolution of the rod, mark the depth on the rod. If you can't get one revolution, go to 6 o'clock (rotation) and mark the rod depth and multiply the depth times 2. That will be a rough measurement of the twist.