Here’s how I did it.
Fill and old case up to the base of the neck with molten lead. When cooled I inserted a long soft lead bullet and chambered it with the rifle pointing upwards.
Then taking a long brass rod I inserted it from the muzzle end until it contacted the soft lead bullet and then pounded on the rod until it didn’t seem to want to go any further.
Ejecting the case leaves the slug behind so you just tap it out with the rod.
That should give you a pretty accurate measurement of the throat which is what you should be sizing your cast bullets to.