Which method can be used to find out strength of reductant/oxidant in a solution? Explain with an example.

Measure the electrode potential of the given species by connecting the redox couple of the given species with standard hydrogen electrode. If it is positive, the electrode of the given species acts as reductant and if it negative, it acts as an oxidant.
Find the electrode potentials of the other given species in the same way, compare the values and determine their comparative strength as a reductant or oxidant.
e.g., measurement of standard electrode potential of Zn2+/Zn electrode using SHE as a reference electrode.
The EMF of the cell comes out to be 0.76 V. (reading of voltmeter is 0.76V). Zn2+/Zn couple acts as anode and SHE acts as cathode.