Well, they guessed, and often guessed wrong.
With lotsa practice in the varmint fields, where you did a lot of range estimating, you were wrong less often than if you didn't practice.
They also used to make split image, hand held rangefinders. Those, once you got used to using them, were almost completely useless. If you didn't practice with them, they were completely useless.
There were also artillery rangefinders- big things you put on tripods. They work. But, you sure don't tote one of those around, as they're 4 feet long, or more, and heavy.
The laser rangefinder is a HUGE advance.