My understanding is that the dangers of excessive headspace have been known by the military for years. The Mauser was a masterpiece in overdesign for strength as well as for diverting the gasses that a case head failure would generate, so that they didn't severely injure the soldier.
The military had armorers and gauges, and the idea was to either repair or remove from service a rifle which had unsafe headspace.
Note that this was assuming the use of military brass (typically thicker than a lot of brass we now use) with military loads (sometimes a little milder than today's loads). It was also using brass that might not have been as consistent in manufacture as can be done today.
Enter the modern enthusiasm for reloading. Now we are concerned about brass life, not just safely firing a cartridge once.
As I understand how things progressed, we now have some fairly well accepted practices for extending brass life, due in large part to experimentation and documentation by reloaders.
If you segregate brass by rifle, and if you shoot a bolt-action, you can use the chamber as a gauge.
If you are setting up to reload for a gas gun, you'd probably better invest in a gauge or two...
I'm not sure everyone who reloaded without tools or knowledge of induced excess headspace avoided case head separations. If they were lucky, they might have had enough knowledge to recognize the signs of incipient head separation and got away with decreased brass life rather than injury.
I think I read about someone who found a guy having trouble with loads for his Garand, and when he dropped them in a case gauge, there was something like 0.016" of cartridge headspace! The Garand is a tough old beast, so all he was doing was wearing out a broken case extractor and testing his eye protection, until someone gave him a hint.