how were the number of protons determined for any element?

[jpet]

The atomic number is the number of protons for an element. How did the number of protons get discovered (historically)? Or how would one determine the number of protons for an element with today's technology?

I am assuming that the technique for deterimining the number of protons for one element would be basically the same as for any other element. If this isn't the case, what are the various approaches needed and why are they different for different elements?