In a way this field, called geochronology, is some of the purest detective work earth scientists do.There are two basic approaches: relative age dating, and absolute age dating.
This is because archaeologists, historians and scientists are always discovering new ways of looking at objects that are discovered.
Relative age dating also means paying attention to crosscutting relationships.
Say for example that a volcanic dike, or a fault, cuts across several sedimentary layers, or maybe through another volcanic rock type.
All living things have the same percentage of carbon-14 in them, but when they die the level starts going down.
Scientists can measure the level of something which used to be alive and work out when it died.