Is it the volume button or the cable?

I have a broken volume down button. I want to do the repair myself. Order a new set of volume buttons and also a replacement battery, so I fix two things at once. But how can I be sure it's the volume button itself and not the audio cable? You can also order this cable, but don't fix what isn't broken, right? Is there a way to be sure it's the button?

Only the volume down (- button) is broken. And when I use the buttons on the headphone cable, everything works fine. (don't know if this helps)

Thanks for the help!

kr,

Richard