The only way of telling a Polecat from a ferret is by DNA analysis. And then, the only difference is in mitochondrial DNA, not the DNA in the cell nucleus. They are therefore, genetically, the same species.
The difference in mitochondrial DNA suggests that ferrets originated as hybrids between two closely related polecat species, or that a small amount of evolution/mutation has taken place, but far in the past. Given the very short generation time for ferrets, there's probably been in excess of 3000 generations of ferrets since they were first domesticated. In human terms, that would be a