Best way to do it is by using an electroscope, a rod (ebonite, perspex, PVC) and the associated material to rub it with (Fur or silk).
Rub the rod with the material to build up a negative charge on the rod, transfer this charge to the electroscope, depending on which kind you have you will see it either spread the two leaves apart or you will see the pointer move (showing it now is negatively charged)
You then shine different wavelengths of light on the electroscope and see what wavelength is needed to release the overall negative charge from the electroscope