The answer is; radiometric dating
This technique relies on naturally occurring radioactive elements in the rocks. The techniques calculate the amount of the ‘parent’ radioactive elements and compare it to the amount of the elements that are products of the element’s decay-‘daughter elements’. The higher the number of products of decay, the older the absolute age of the rock layer will be. The half-life of the radioactive element must also be known for the age of the rock to be postulated.