They did it by eye and off charts based on typical things like a human, human head, car, etc. Fixed sized things. You say accurately. It wasn't really done accurately in the way we understand it today. Rangefinders can measure to the metre or less. Old-school snipers also didn't have scopes all the time, just iron sights. Not always, but often. It wasn't until World War One that scopes began to get used and developed.
