In the area west if the Appalachians, they made what were called "tomahawk claims". This was done by marking trees as the boundaries. This was a very inexact practice and as areas became settled and courts appeared, they were tied up in courts all of the time. You also have to remember that with a sparse population there was plenty of room so they didn't build right next to each other.

they used natural boundaries like rivers or mountain ridges.

or used standard navigational tools to follow straight lines on the map: compass for direction, polar star for N-S latitude, accurate clock for E-W longitude

They'd literally hammer numbered stakes into the ground , hence "Stake your claim". The exact extend of the claim was usually defined by distance from the stake and farmers would usually fence off the extent of the claim as soon as they could.

There have been a couple of different survey systems over the years. A lot of Western North America was laid out using what are called Survey Townships - basically, you make a square that is 6 miles long by 6 miles wide. That gives you 36 square miles. An area of 1 square mile is called a "Section," so within a survey township you have 36 sections. Sections could be further subdivided into quarter-sections (160 acres,) or even smaller units at will. This is also why if you look at the map of counties in most states west of the Mississippi, most of the counties are squares.

How did you know the boundaries? The General Land Office, which was an office of the US Government during the time of westward expansion, hired surveyors to mark out the boundaries. They would literally hammer large spikes into the ground at the corners of properties.

In fact, to this day, you can often see property stakes. Nowadays they're more likely to be made of wood, but some of the old iron pins are still out there.

My parents house has large rocks for 2 of the property lines, one crossing a hill and the second through a swamp/marsh wooded area. Probably about a dozen or so boulders, about the size of an 8 year old kid, probably 200lbs each. There are no obvious trails or paths to the rocks dropped in the swamp, would love to know how they got them there, lined up so perfectly.

A very good point that sometimes they straight up didn't know for sure. Some claims overlapped or were worded so vaguely that the interpretations had to be settled in a court.

In Australia? It was very iffy. You could be given land by the King but it would take months for that info to get to Oz. In that time, the Governer of Sydney (the basic whole of Australia back then) could have given someone else the same land under the authority of the King which also could not be disputed. It would have been the word or the King against the King. Given that the currency in the early settlement days was rum, essentially people just got drunk and bitch slapped each other, or they took convict labour and the First Nations people and forced them to build fences(ish), or they died and squatters rights sometimes kicked in or the whole lot was forgotten.

It's possible that the rocks were there first and repurposed as boundary markers afterwards. You often see rivers and streams used this way.

That being said, it's entirely possible that someone deliberately used an oversized rock to mark the boundary because they didn't want something that could easily be moved. Using stones is an old method, though, dating back to antiquity.

George Washington was a surveyor, and his job was to mark the borders of purchased properties, so it was a known thing in the early 1700's.

The Texas Rangers started as surveyors when white settlers began homesteading land in the area that supported the Commanche tribe.

It was profitable work, but it ended up being done by men who could defend themselves from criminals, Mexican bandit gangs, and hostile Commanches.

They evolved into law enforcement when other policing organizations refused to go there until it was more settled.

Many markers are still metal. Wooden stakes may be used for temporary markings, but the actual reference points those are based on will be designed to be permanent.

You can probably find a variety of survey markers in the pavement in your neighborhood, if you know what to look for. For minor locations, they're usually a small pin driven into the concrete or asphalt.

For more major stations, they'll have a circular disk with information about the station. Even more rarely, you'll find geodetic survey markers that contain information about the actual curvature of the Earth at that point, since even the shape of the Earth can vary enough to be significant at large scales.

https://en.wikipedia.org/wiki/Survey_marker

The US employed lots of surveyors. As the government moved west, so did the surveyors. It was an important government (and private) job and the law for it dates back to the 1780s - it was passed by the Continental Congress before the revolutionary war had even ended.

They developed a plan on how to survey the Northwest Territories (which stretches from Ohio to Minnesota). The US created a government post of the Surveyor General of the Northwest Territories in the 1790s. They were responsible for setting markers and would go out west with lots of survey equipment and lay down where all the key points are. There are a ton of "County Line Road" roads out west because the road would end up following the surveyor line.

Before this was systematically laid out, surveys would be based on natural features, like "from the large bend in the stream 1/2 mile from the junction with the Big River, 400 yards to the large, solitary walnut tree." But that's not a great system, as rivers move, things erode, trees die, etc.

In some places, the first surveyor would inscribe letters and numbers into a tree near where a corner was. https://glorecords.blm.gov may help you find early documents for your area.