a fossil
Well, most of the time geologists find fossils of plants and animals, and then study them to find out how long ago the plant(or animal) lived.
There are a lot of things you can tell form animals that lived long ago. Fossils give us many atributes to that animal. like the shape teeth what they ate and many other things
You can find out because you can see what the animal had small feet that means it was small or if the animal has big feet it was a big animal
Fossils are bones stick together and they can be a person or animal and if it was a animal you can tell what animal is it where it lived and by the condition you can say it came form a long time ago or present day.
100000000 years ago
nothing but humans lived 2000 years ago
the remains of a living thing that lived a long time ago is called a fossil.
About 500 to 600 years ago
yes
69 millon years ago
65 million years ago.