The field of nuclear physics has established the radioactive decay series for radioactive elements (see here as well).
Unlike Uranium 238, Uranium 234 is not primordial nuclide. It is a indirect decay product of Uranium 238.
By knowing what elements, and their isotopes, are present in rocks and how much of these isotopes are present, combined with the radioactive decay chain it is possible to determine the amount of Uranium 234 that was once present when the rocks formed.