Now, paternity is presumed when a marriage is intact, however after the Pennsylvania Supreme Court decided Brinkley v. King, a third party may now assert paternity when a marriage is either no longer intact or if there was never a marriage, to begin with. The third party still may not attack paternity if the marriage is intact. This barrier to paternity is known as an estoppel. If a third party attempts to attack the paternity of a child who is part of an intact family, they will be stopped.
If you are the presumed father and you begin to doubt that you are in fact the father of the child, there are a few things you can do. First, if you are on the birth certificate yet not married to the mother and she filed for support, you may refuse to sign an acknowledgment of paternity. If your doubts arise once the child has become older, the process is trickier. If evidence surfaces that the mother of the child has been defrauding you, it is important that you stop filling the role of a father. By cutting ties, you are shielding yourself from claims from the other party that you knew you were not a dad yet still acted as dad anyway. If you remain in the role of dad, this could result in you still being considered dad indefinitely and still being required to pay support (if so paying).
If you have stopped acting as a father, you may petition the Allegheny County Court of Common Pleas, Family Division through a motion to see if you can be granted a paternity hearing. If granted, the family court will determine in this hearing if you are to be granted a blood test and a final judgment on the paternity of your child.