How can scientists measure the standard reduction potential of a half-cell
The standard reduction potential of a half-cell is measured by pairing it with the reference electrodes (primary-Hydrogen Electrode, secondary-Calomel Electrode) to form a cell. This is measured from potentiometer. Once the value of the reference electrodes is identified, the standard reduction potential of the unknown half-cell is identified. Check out http://chemwiki.ucdavis.edu/Analytical_Chemistry/Electrochemistry/Redox_Chemistry/Standard_Reduction_Potential for more information.
Well you need to have a starting point somewhere. The one that was chosen was the one of hydrogen gas as the 0 point, hence the 0V potential for the reaction. The other potentials or thus just measured in regards to the hydrogen potential.