You can get the hints of mitochondria to function by western blotting of mitochondria-associated proteins: TOM20, ATP5A you can also observe mitochondria structures using an electron microscope. However, in order to claim disfunction, you need to perform direct measurements of the cellular oxygen consumption and glucose utilization such as the Seahorse instrument that will show metabolic changes in the cells taken together this data can result in the robust claim of mitochondria dysfunction anything less may be criticized not robust enough.
Also analysing the mitochondrial membrane potential (MMP) with TMRE or JC-1 shows the status of mitochondria. The reduction of MMP displays the decrease of mitochondrial activity