I know how to measure the concentration of free thiols, which is reflective of the redox-potential of cellular compartment such as the cytoplasm or a lysosomes.
- What other methods exist?
There are many complexes of d-elements, that can reversibly lose/gain one electron, changing their color in process. They are known as Redox Indicators. Some of them are stable enough to be used in vivo.
Electrochemistry; using microelectrodes. So, basically, place one microelectrode inside the cell, and another (the reference electrode) just outside the cell, and then use a potentiostat to measure the potential between them. Reference.