Interestingly, with an oscilloscope you can see the harmonics in all their gory detail :)

Actually depending on microphone or instrument interface, you can see stuff that's beyond the range of hearing too.

Also, on a low-frequency long-string like an upright bass, if it is bowed at the halfway node, you still hear mainly the fundamental but the second harmonic is naturally emphasized more than usual, and you can also see half the string making its contribution as pictured, with the naked eye.