LEDs, CFLs, even incandecents, all have vibrations that are audible when you pull audio out of the solar cell. Sunlight, not much. No audible vibrations at all.

IR remotes are fun. For the last show I played, I put a couple layers of garbage bag over the solar cells to make an IR pass filter to block the stage lights from interfering.

Really, give it a try.

If it goes from sound to silence in the first case and from silence to sound in the latter, you can be reasonably sure that light has an effect.

If you want to rule out other radiation, you can try shielding the contraption.