For cartridges that reference on the shoulder, like 30-06 or 8mm mauser, or 7.5mm Swiss, place one piece of masking tape on the base of an unfired 7.5 Swiss cartridge and see if it chambers. (It should). Place a second piece of masking tape on the base and it should be very difficult to chamber. Place a third piece of tape and it should be impossible. Masking tape is about 0.004" thick.
If a rifle headspaces long, consider if you would be willing to handload for it. I had a BSA 30-06 that was at least 0.015" long that still had great rifling and shot great. I just couldn't burn factory ammo in it.
For rimmed cartridges like 7.62x54R, 30-30, etc, the base references the rear of the cartridge so they are very tolerant of headspace. The thing that makes excessive headspace dangerous is that if a cartridge fires when the base is unsupported by the bolt, the brass is too weak to support the pressure alone and will rupture, sending hot gas into your face. A rimmed cartridge always has the base supported by the bolt, so a few extra thousandths ahead of the cartridge are OK.