How do we determine the age of ancient bones and artifacts from archeological digs. The most common method is by carbon-14 dating. The following video describes the theory of behind Carbon-14 dating and how it works.
https://www.khanacademy.org/science/ap-biology/natural-selection/evidence-for-evolution-ap/v/carbon-14-dating-1
A number of different isotopes with very long half-lives are left over from the original formation of the Earth. These have been used in determining and describing geological ages.