How are physiological reversal potentials (or ion concentrations) of neurons measured?
There are various methods to determine the intracellular ionic concentrations. One method is to register action potentials by the fowling method: inject a Voltage Active dye VAD (something like a -2di Anepp or s similar dye) into the cell or tissue and using an Optic Mapping System register the Action potentials and then determine the intracellular ionic concentrations by Nernst equations.

We could also register Action potentials by "ion-sensitive glass electrodes method" by introducing ion-sensitive glass electrodes directly into the interior of the cells. The problem with these electrodes is that they cannot register very low ionic concentrations especially of Calcium. And so the above method is superior using  VAD.