State of charge determination within electrochemical systems, such as flow batteries, can often be difficult to measure, particularly in an in situ manner. Methods for assaying the condition of an electrochemical system can include: interacting electromagnetic radiation with a first electrolyte solution at a location within the electrochemical system, the electromagnetic radiation being delivered through an optical material configured to exhibit attenuated total reflectance at an interface between the optical material and the first electrolyte solution; receiving at a detector electromagnetic radiation that has interacted with the first electrolyte solution via one or more attenuated total reflectances within the optical material; and measuring an absorbance of at least one of an oxidized form or a reduced form of a first coordination compound within the first electrolyte solution via the electromagnetic radiation that is received at the detector.