Instead, other methods are used to work out a fossil’s age.
These include radiometric dating of volcanic layers above or below the fossils or by comparisons to similar rocks and fossils of known ages.
Different methods have their own limitations, especially with regard to the age range they can measure and the substances they can date.
A common problem with any dating method is that a sample may be contaminated with older or younger material and give a false age.
Tenmagnet drops in a nice chapter on the lost art of phone game.