What is a good method to measure the redox potential of a cellular system? 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.