What defines a mutant is the meta-gene complex, also called the (MGC). This is a collection of 18 changes scattered over a number of chromosomes. Most people who have the meta-gene complex (at the last survey this was about 1/6th of the population) aren't "mutants". That requires activating the complex. Nobody knows how that happens. The whole concept of a "mutant" is, therefore, rather fuzzy. It's solid enough for the mass media and politicians, but once you get beyond the rabble rousers it becomes less and less precise.
It should be noted that while there is no "mutant test", it is possible to genetically test to see if the meta-gene complex is active. While there are people with an active complex who don't have super powers, all mutants with measurable abilities have an active complex.