The best methods are what we call geochronological methods. Basically these methods work because certain elements are radioactive elements. A Good example is Carbon. The element carbon comes in three forms or isotopes... carbon-12, carbon -13 and carbon -14.
Now carbon-14 is UNSTABLE. This means that if you had a lump of carbon-14 and set it on a table, after 5700 years half of the carbon 14 would decay to carbon -12. Then after another 5700 years half of the remaining carbon-14 would decay to carbon-12. Etc.
So let us say we want to date some wood or charcoal we find buried along with some deposits of say human artifacts (like tools that early humans used).
We can collect some charcoal from the same layer and determine the ratio of carbon-14 to carbon 12. This ratio we can use to determine when the plant died. That is how long ago the plant stopped taking in new carbon-14. since that time, since the plant was not growing and absorbing new carbon-14, the carbon-14 it had when it died will have decayed to carbon 12.
I hope this helps. You can go to web and Google CARBON DATING to learn more about this subject.
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