Fission track dating counts the number of tiny holes made in a crystal by the decay of radioactive atoms. Thus, there’s no question of what was already in the crystal.
As already mentioned, some radiometric dating methods rely on an element that does not form part of the crystal structure of the mineral, so the starting amount is zero. Most elements can show up in various concentrations in many minerals, though. In this case, isochron dating methods are used. For example, for rubidium-strontium dating, you measure the amount of 87Rb, 87Sr, and 86Sr in several minerals from the same igneous rock. The chemical differences between Rb and Sr leads to different amounts of each in different mineral crystals. But 87Sr and 86Sr are practiically identical. Once they are locked into a crystal structure, over time some of the 87Rb atoms decay. A crystal that started with more 87Rb will have more extra 87Sr over time. Thus, comparing amounts in the different minerals will allow identification of the starting amounts of 87Rb and 87Sr.
But the efforts at denying that radiometric dating works ignore the fact that anyone since the 1770’s aware of geology knows that the earth is old. Young-earth models cannot credibly explain the series of geological layers and all of the events that they record. Likewise, young-earth claims about history rely heavily on believing “Enlightenment” propaganda rather than serious historical research. Old-earth ideas were present from the early Church; most of those discovering geological evidence about the age of the earth in the 1600’s and 1700’s were Christian.