Do you mean DNA testing to:
1) Determine who the mother is? (e.g. a child put up for adoption but who suspects a given women is his/her mother, a not uncommon situation in Argentina where some Jewish babies were adopted by non-Jews after their mothers were disappeared)
2. Determine if a woman became pregnant by someone other than her husband?
3. Determine some sort of more distant relationship?
I believe the first would probably allow DNA testing to prove someone is Jewist. I believe the third would have no halachic significance ever. I don’t believe the second would be accepted to prove someone to be a mamzer absent more significant evidence but I’m sure there is debate over the matter (with the more traditional being the least willing to accept scientific evidence).