Detecting thin aluminum phosphate coating

For electrolytic capacitor, the aluminum foil always reacts with water to form oxide. So, some manufacturer will put the foil in phosphoric acid to form a protective layer (aluminum phosphate). So, how to detect this layer? what kind of method can be use?

I got a sample with a very thin (about 10 um) coating of aluminum phosphate. Does anyone know how to detect this coating?

Surface phosphorous on aluminum is detectable and measurable using EDAX - also, the aluminum phosphate is a dielectric and a measurement of impedance through the aluminum will correlate with the amount of aluminum phosphate on the surface- are commercial samples of the treated foil available?