H2S2O8 has sulphur in the +6 oxidation state, hydrogen in the +1 oxidation state, the two O's between the two S-atoms are in the -1 oxidation state and all other oxygens are in the -2 oxidation state.
Whenever there is peroxy linkage , sulphur is always in its highest oxdidation. state i.e. +6
so here, oxidation. state of sulphur is +6