Sequecncing fossils due to the layers of rock built over time. The older the fossil, the deeper they are in the rock, The younger fossils are towards the surface.
This method helps to connect fossils found at different location in similar rock. By using a marker fossil, usually an organism with a hard shell, a chronology of fossils can be made.
Finding the half-life of isotopes found in fossils from radioactive decay. This can put an actual date and age on the fossil.