Depending on how deep the rock was in the ground or the sediment around it scientific researchers in the GEO category can study the rock and find out.
Looking also at the bones or dirt you can tell by the clay or even the aged looks of it's outer physical appearance.
If you look at the inside of a rock , Geologists can run tests on it and find the actual age of the rock.
I really hope that this helps you out a lot.
