It took a canal surveyor circa 1800, William Smith in England, who noticed that he could map out great tracts of rocks on the basis of their contained fossils.
This lead to the recognition of one of the principles of geology, i.e., stratigraphy, older rocks lie below younger rocks and that fossils occur in a particular, predictable order.
The next step was for geologists began to build up the stratigraphic column.
One fact was soon clear, no dinosaur record could be found to coincide with a human fossil record.
So it was apparent that there was some kind of 'progress' going on.
It was all clear when in 1859 Charles Darwin published his "On the Origin of Species".
The 'progress' shown by the fossils was a documentation of the grand pattern of evolution through long spans of time.
At the oldest, or deepest layer of rock there was no record of fossils, but then they noticed that simple sea creatures were found at the next higher level, then more complex ones like fishes at the next higher level and so on.
Next came life on land, then reptiles, then mammals, and finally humans.
Chemists can measure the half-life of these elements, which is the time it takes for half of the radioactive parent element to break down into the stable daughter element.
Then by comparing the two proportions of parent to daughter elements in the rock sample, and knowing the half-life, the absolute age can be calculated.
Besides the order of fossils in the rocks, another method is the use phylogenetic trees.