If the case body was perfectly round, you could put it in a V block then measure neck runout or centering with a dial indicator. But neither cases nor chambers nor dies are perfectly round. They're typicaly no worse than 1/1000th inch out of round at best, but some cases are much more so.
As cartridge brass in cases ain't the same thickness all the way around, It won't spring back to perfectly round after sizing or firing. That out of round condition will effect bullet runout numbers when the case body right behind the shoulder is where the front spinner rest is at. Meanwhile, the back of the case rests in another V block or something like that. Cases laying a single V block that contacts the case from its back to the front will have all sorts of variables introduced into the runout numbers. Get a V block and dial indicator set up to measure how round something is, then check your fired and sized cases right behind the shoulder as well as the pressure ring about 1/4 inch ahead of the head.
If you have a cartridge spinner that rests the back end in a V block and its shoulder centered in a round holder (nylon washers work very well), then the case is held the same way as it is in the barrel's chamber. You could use one that has a V block that can let the case shoulder rest on it at mid point on the shoulder. Measure runout at the front of the bullet about 1/10th inch back from its point as well as on the case neck right in front of the shoulder and at its mouth to see how much runout there is all over. I had to make my own 'cause there was none on the market decades ago.