Reloading .303 British : Reloading 303 with crimp

Easy way to tell is "smoke smudge" the case then run it into the die as you would when loading normally. the soot should be removed as the die resizes then expands the neck inside & out.

That will tell you the amount & position of the resizing die's contact, simple inside & outside diameters measured with a caliper will tell you the diameters.