you look for landbased tree markers. those that are century old mango trees, balite tree or fire tree. it implies there is a chamber or dozens of chamber in your land. look also for buri trees or any century old fruit tree. but the only trees that signifies an entrance to a chamber are mango trees, balite or acacia tree, or fire tree.
as for the stone, you really cant tell the difference if it is natural or not. they all look the same.