How can scientist measure the standard reduction potential of a half cell

It is possible to determine the standard reduction potential of half cell by using hydrogen electrode as a reference electrode.

The standard oxidation potential of hydrogen half cell, under standard conditions, is 0 v. Hence, when an unknown half cell (say X/X-) is clubbed with hydrogen half cell, it directly gives the half cell potential of electrode of interest. 

Such electrochemical cell can be represented as follows:

                                                     Pt/H2/H+//X-/X

Here, Eo cell = Eo X/X-  -   Eo H2/H+

But,  Eo H2/H+ = 0 V

∴ Eo cell = Eo X/X-