The three particles in an atom are compared using relative charge and relative mass. A proton has a relative charge of +1 and a relative mass of 1. A neutron has no charge, so its relative charge is 0, and it has the same relative mass as a proton, which is 1.
An electron has a relative charge of -1, the opposite of the proton. Its relative mass is very small, about one two-thousandth of the mass of a proton, so it is often treated as almost zero.
You can use this to identify a particle. Anything with a relative mass of 1 and a charge of 0 is a neutron, and anything with a very small mass and a charge of -1 is an electron.