Determine the number of protons in the atom. Due to their extremely small size, this number is likely a given; protons cannot be observed by a normal microscope.
Look up the atom by the number of protons it has. Atoms on the periodic table of elements are organized by their atomic number -- the same as the number of protons.
Subtract the atomic number from the atomic mass to find the number of neutrons in a stable version of that atom. Electrically neutral versions of the atom will have the same number of electrons as protons.