Using relative dating the fossil is compared to something for which an age is already known.
For example if you have a fossil trilobite and it was found in the Wheeler Formation.
The example used here contrasts sharply with the way conventional scientific dating methods are characterized by some critics (for example, refer to discussion in "Common Creationist Criticisms of Mainstream Dating Methods" in the Age of the Earth FAQ and Isochron Dating FAQ).
I thought it would be useful to present an example where the geology is simple, and unsurprisingly, the method does work well, to show the quality of data that would have to be invalidated before a major revision of the geologic time scale could be accepted by conventional scientists.
There are two main methods determining a fossils age, relative dating and absolute dating.
Relative dating is used to determine a fossils approximate age by comparing it to similar rocks and fossils of known ages.
Each original isotope, called the parent, gradually decays to form a new isotope, called the daughter.
Each isotope is identified with what is called a ‘mass number’.