The elders used a device called a plumb line. It used a weight suspended by a string to establish a perfectly vertical line (“plumb”).
It was a foundational tool in construction and surveying for ensuring walls, posts and structures were straight up-and down. It is still used today in developing regions.
You can make one with a length of string and nearly any weight sufficient to stretch the string.
Advantages of this ancient technology include: high precision (often surpassing lasers), simplicity, and low cost.