You'd use a plum bob within an isosceles triangle, like the square level here from an Egyptian tomb.

You can start with just a taut piece of string or thread- that will give you a straight line. Then you pare and plane two boards, checking with the line, until they're straight. Put them together with a nail to make a right angle square, adjust that angle by scribing a straight line, then holding one leg of the square against that and scribing another line with the upright, then flip the square and scribe from the other side. Adjust the angle to divide the difference, put another nail in the corner, and you have an accurate square. Layout an equal distance on each arm of the square, run another straight board across, connecting them, and you have the Egyptian square level shown on the website, though without hieroglyphics. ( This is just doing it from scratch, of course if you already have a square you can make another one from it without doing all this)

For long distances: put a notches or loops of wire into that connecting piece, so you can sight along the top of it accurately. Now you have a surveyor's transit- if the plumb bob is pointing right in the center of the connecting piece, it's level. So if you hold it on top of a one-meter tall post and sight along it, you should be able to have someone drive another post until it's right in the sights- that will be one meter as well.

There are also simple things like pouring water on something and seeing which way it runs off or puddles, or setting a bowl of water on something to see if it tilts. The Romans had water levels on their transits.

A very nice website with more info on Roman surveying tools and methods is here