What is an isotope and how can you tell them apart?

A number of atoms of some elements have been identified which have the same atomic atomic number but different m number.for example take the case of an hydrogen atom,it has three atomic species:-protium(hydrogen atom with atomic number 1 and m number 1),deuterium(hydrogen atom with atomic number 1 and m number 2),and tritium(hydrogen atom with atomic number 1 and m number 3).to tell them apart remember that the atomic number are same but m number are different.