Isn't the primary experimental argument beta decay from that link? A nucleus can emit a positron, and observably loses nuclear charge equal to one positive electron.
So by a pretty simple inferrence you could conclude the proton has a positive in it, hence the charge (it of course isn't literally like this for other reasons though).
And since we also observe antiprotons, the opposite can clearly apply.
So by a pretty simple inferrence you could conclude the proton has a positive in it, hence the charge (it of course isn't literally like this for other reasons though).
And since we also observe antiprotons, the opposite can clearly apply.