We know that for the same isotope, J will remain unchanged. Additionally, for minimal F there is a minimal energy (F=|I-J|). Thus if we measure the change between the lowest energy state to the value before splitting we obtain the matrix element. with other words, you can calculate F since you know I and J is just the same as before, you can measure the matrix element and all except the magnetic moment is known in the prefactor because of the first measurement. This means we can insert the known values and calculate the magnetic moment easily.