Brian Monetti, Science is Awesome!
Generally the mass of a atom will be given in Atomic Mass Units (amu.) One amu is roughly the mass of a proton or a neutron. If you know the mass of the atom in amu's, you can subtract the number of neutrons from it to find the number of protons. You can be more precise by subtracting the exact mass of a neutron in amu's (
Alternatively, if your mass is in another unit (kg, g, eV) you are best off converting to amu's and using the method I described above!