This is why Manx genealogy is rewarding in that it is usually fairly straightforward to trace the female line, due to all those "aliases".

As regards the DNA the family groups participating can soon tell if one result does not fit in with the rest, as apparently there will be a very small number of genetic mutations (hopefully) in many generations on the Y Chromosome. Therefore unless the illegitimacy was before the first branch ocurred in the lineage being traced, this will show up.