Simply by measuring a Q-H curve of the specific component, i.e. top measure the flow rate through the porous component and measure the pressure drop to force this flow through the component. Normally, with a quite low porosity the flow should be laminar and there should be a linear relation between pressure drop and flow rate. Porosity P is found from Q_air=P*A*delp, where A is the area and P the porosity. With microscopic or X-ray images this can be use to measure internal parameters describing the porosity.