The majority of the time fossils are dated using relative dating techniques.
Using relative dating the fossil is compared to something for which an age is already known.
Sometimes beds of rock can turn over the other way, so be very cautious when relatively dating rocks!
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used commonly when looking at the relative order of geological events.
What can complicate relative dating is when the strata is not the right way up!
For example, based on the primate fossil record, scientists know that living primates evolved from fossil primates and that this evolutionary history took tens of millions of years.
We have rocks from the Moon (brought back), meteorites, and rocks that we know came from Mars.This method is a bit vague, which is why modern scientists have developed many methods by which to determine the absolute age of Earth materials.The absolute age of an Earth material is a measure of how old it actually is in years.An absolute age is one determined usually by mass-spectrometry where an isotope is measured and then an age can be calculated (a very very basic explanation).So in the end you can say this fossil is 50 thousand years old (always with an associated uncertainty).