We know that Oxygen usually has a charge of -2, and given that there are 4 Oxygen atoms in S2O4, the charge contributed by Oxygen is 4 x (-2) = -8. Since the charge of S2O4 is -2, we know that the total charge contributed by sulfur is +6. +6 divided by 2 Sulfur atoms gives you the oxidation state (or number) of S: +3.