Well I don't think you have the proper test equipment to do that.

But this might be a simple way to check.

Using your default mp3 player. Select a song and play it. Turn the volume all the way up on the mp3 player, also turn the volume all the way up on your speakers. Cover ears if needed and warn anyone around.

Watch your display. If it fades in and out with the rythym of the music, that is an indication that the power supply is being stressed.

It might also duplicate your problem.