To find the oxidation number first you should know about oxidation number.
Oxidation number, also called Oxidation State, the total number of electrons that an atom either gains or loses in order to form a chemical bond with another atom.
In Peroxydisulphiric acid (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