Get a few de-primed once-fired cases and a gage and get to work. Here’s a Forster Datum Dial gage. Works well and works for all standard-architecture bottleneck cartridges, as does the Hornady LNL. Each or either gives a “real” headspace number (although it’s not perfectly congruent, without mathematical manipulation, to the figure from a headspace gage used for chambering; that doesn’t matter though: as long as the gage is zeroed it shows the difference, and that’s what matters). By the way, the old standard “drop-in” style case gages might keep ammo safe, but won’t provide this sort of detail in information.