If the oxidation states of the atoms in the reactans are different from the oxidation states of the same atoms in the products then it is an oxidation-reduction reaction.
Both atoms H and F in the reactants have oxidation states 0.
That is a basic rule: any atom alone or bonded to the same kind of atom has oxidation state 0.
The oxidation states in HF are: H: +1, and F: -1.
So, the H increased its oxidation state, which is that ii is oxydized ; while F reduced its oxidation state so it is reduced.
