In relativistic theories the effect of nonconservation of simultaneity can be separated from that of the Lorentz-Fitzgerald contraction. Since with absolute synchronization simultaneity is conserved, we show that a simple kinematic test may discriminate absolute from Einstein synchronization, settling the century-long debate on the conventionality of the one-way speed of light. An immediate consequence is that Einstein's postulate of a universal light speed can actually be tested and the unique physical meaning of special relativity (SR) is restored. Only an experiment can corroborate SR or identify the preferred frame of reference.