I don't know how to measure it. I'm relatively new to muzzleloading
With any rifled bore, not just muzzleloaders, all you need is a cleaning rod with a snugly patched jag and a tape measure.
Twist rate is one turn in so many inches. A 1:10 twist means the bullet, and consequently the patched cleaning jag, will rotate one full turn in ten inches. Fast twists like that are easier to measure. Start the jag in the bore, mark TDC on the cleaning rod, mark a starting point in reference to the muzzle or the action, whichever direction you're going. Push the rod in until your TDC mark has turned one full turn. Mark the rod again at the same reference point. Pull rod and measure between the two marks. That measure will be the twist rate.
On really slow twisted bores like 1:48 or 1:66 muzzleloader barrels you have to math in halves or quarters, because you don't have enough barrel length to get a full turn of the cleaning rod. Use the same method but mark TDC and mark exactly 180 degrees from that, which will give you half a turn instead of a full turn. Push your rod down the bore until the "half turn" mark is now on top. When you measure between the two reference marks you will have 24 inches for a 1:48 twist, or 33 inches for a 1:66 twist.
If you still don't have enough barrel length then you'll have to measure a quarter of a turn. Mark TDC and mark 90 from that. When you measure between the two reference marks 12 inches will be a 1:48 twist, and 16.5 inches will be a 1:66.
An experienced gunsmith or gun nut who knows what common muzzleloader twist rates look like to the eye can simply look and see what it is. 1:66 is a starkly slower twist than 1:48, and a 1:24 is obviously faster than either of those.