What is the charge of the nucleus of a phosphorus atom, and how do you know?

The phosphorus nucleus contains $15$ $\text{unit positive charges}$. Of course the neutral atom is also associated with 15 electrons, so the net electronic charge of the atom is ZERO.
How do I know this? Well, I have access to a Periodic Table, for one, and it says unequivocally that $Z = 15$ for the phosphorus atom. $Z$ is the number of protons, massive, positively-charged nuclear particles; $Z$ is more commonly called the $\text{atomic number}$, which UNEQUIVOCALLY defines the identity of the element.
$Z = 1$, hydrogen; $Z = 2$, helium;......................$Z = 15$, phosphorus; ...............$Z = 26$, iron; etc.