Answer
Use the Franck-Hertz effect.
Work Step by Step
As discussed on pages 1293-1294, the Franck-Hertz experiment provides direct evidence for the existence of atomic energy levels.
Electrons with a kinetic energy of 4.9 eV or greater caused mercury vapor to emit UV light. This shows the existence of an energy level in Hg that is 4.9 eV above the ground state. The emitted UV light is of the expected wavelength of 250 nm.
