What are the best resources for confirming Jewish ancestry?

My wife’s family is polish. There is a sort of “family secret” that her grandmothers mom (great grandmother) was Jewish on her moms side. I’m not Jewish but if I understand correctly this would make my wife Jewish according to halakhah.

The issue is we have no way to confirm or deny this. My wife’s grandmother refused to speak about it and is now deceased. I believe there was a lot of repressed trauma after having lived through the pogroms in the early 1900’s.

Is there any way of conclusively figuring out if my wife’s great grandmother was Jewish?

We’ve both done dna tests and both have partial Ashkenazi DNA (not that that means anything conclusive).