A legal document birth certificate and a DNA test would prove it.
Birth certificate won’t work. DNA will.
In my past life I once got involved in establishing maternity on a child support case. That was purely unique but it is also true. I nearly fell out of my seat when my investigator told me about it. Seems the girlfriend used her married boyfriend’s medical insurance to birth the child on a military base. When the “legal” mother was confronted she threw a fit and no wonder. It wasn’t her child.
That would be rich wouldn’t it? Especially given the whole Obama COLB story.