Any decent gunsmith has a guage for measuring interior bore diameters and could tell you about any jug choke in the barrel. But why are you worried about this on a muzzleloader? Any ball you stuff in through the muzzle is already past the tightest restriction and will comeback out the same way. Jug choking typically involves opening the bore a little larger for a few inches behind the muzzle area so the shot column sees the muzzle area as a constriction just as in a normal shotgun choke and having a transition from the new larger interior dimension to the original bore at the muzzle.
You can also probably detect jug choking if you take one of the small led flashlights and slide it into the bore. If the light is too bright cover the bulb with some semi transparent material. The jug choke should appear as a shadow line within a few inches of the muzzle.
If you have an old muzzleloader which hasn't had the breechplug pulled to examine it's condition that would be a good safety idea anyway. This needs some care on shotgun barrels as they are thin enough in some cases to be damaged if you don't put them in a vise with proper clamping.
