How to determine electrolyte concentration in solutions?
Electrolytes turn water into a conductive solution.  Conductivity is affected by how free the electrons are to move.  Conductivity (which is the reciprocal of resistivity) is affected by by concentration, temperature, and how much charge the ions carry.  Conductivity alone will give an equation with both ions and concentrations; however, this equation will have multiple solutions. For some fun understanding this phenomena look at this this link.   

Conductivity is different than voltage.  Voltage in this situation is likely referring to a half-cell reaction with a standard hydrogen electrode in a galvanic cell.  Using this approach has the same issue with a multiple solution equation.  

Electrolytes are typically ions.  Ion concentration is measured with ion chromatography.  Ion chromatography involves separating the ions from the solution based on various properties to perform a measurement. It is explained here, here, and here.

When electrolytes of drinks are measured it is important to include if the electrolytes are bio-available.  Bio-available electrolytes depend on the activity performed.  An activity such as sports uses a different set of electrolytes than sitting.  

Testing bio-available electrolytes requires more work than analytic chemistry. Analytic results both the drink and bodily fluids of a population doing a certain activity are need to draw conclusions about the electrolyte composition of a drink. 

Reading a patent for a sports drink ions include $\ce{Na+}$, $\ce{K+}$, $\ce{Mg^2+}$, $\ce{Cl-}$, $\ce{SO4^2-}$, $\ce{PO4^2-}$.  The patent also includes citrate, sucrose, dextrose, ascorbic acid and pyridoxine as these are known to affect the uptake of electrolytes. 

If you have access to a university library the values of these compounds for orange juice are available.