Good point, probably one way would be to test the capacitors at two different freq. high an low plus measuring the capacitance (computing the ideal values from capacitance and freq to get the ideal reactance and inductance) and than analyze the outcome comparing all the values and decide upon this result for a proper verdict of what the cap is good for and where it can be used, would be nice to have some capacitors with the same value and different ESR and ESL to test for this variable, the sketch could be modified to use different freq. This gives me an idea of a sketch where we could analyze the capacitor's response on a frequency range and plot the value and compare this to the ideal capacitor.