Direct radiometric dating accuracy sprung the dating game ign
We know it is accurate because radiometric dating is based on the radioactive decay of unstable isotopes.
For example, the element Uranium exists as one of several isotopes, some of which are unstable.
When an unstable Uranium (U) isotope decays, it turns into an isotope of the element Lead (Pb).
Thus the Cretaceous–Paleogene extinction (when the dinosaurs died out) - previously dated at 65.0 or 65.5 million years ago - is more accurately dated to 66.0 Ma.
This technique allows the errors involved in K-Ar dating to be checked.
Argon–argon dating has the advantage of not requiring determinations of potassium.
This gives geologists great confidence that the method correctly determines when that rock formed.
Hope that helps, and please ask if you'd like more details! I think that I will start by answering the second part of your question, just because I think that will make the answer to the first question clearer.