There are two primary ways of dating used by geologists: relative dating, radiometric dating and fossil dating.
Dating rocks and fossils
Excellent page here
Great animation here
A Framework for K-12 Science Education: Practices, Crosscutting Concepts, and Core Ideas (2012), from the National Research Council of the National Academies.
By the end of grade 12. Radioactive decay lifetimes and isotopic content in rocks provide a way of dating rock formations and thereby fixing the scale of geological time.
ES.3 Earth’s History: Relative and Absolute dating. Students understand that various dating methods — relative and absolute — have been used to determine the age of Earth.
Suggested Connections. Between Earth Science and Other Disciplines: Evidence of Common Ancestry and Divergence (LS.1.1); Living Systems and the Physical Environment (LS.3.1); Nuclear Chemistry (C.1.6); Nuclear Interactions and the Conservation of Mass–Energy (P.2.3)
Knowledge of radioactivity helps them understand how rocks can be dated, which helps them appreciate the scale of geologic time… Scientific evidence indicates that some rock layers are several billion years old. 4C/H6** (BSL)