If you’re referring to Mary’s own birth, she was not delivered by a virgin birth. Her mother conceived her naturally (although without sin) and delivered her normally. If you are referring to Christ’s birth of the Virgin, Mary herself could have testified that she gave birth to Christ without having ever experienced marital relations (cf. Luke 1:30-35). Her own saintly character would have been verification enough of her claims.
To understand this, we must remember that there was a time when people of good character did not dare to cast aspersion on the good character of others without solid proof of bad character. To question someone’s honor or integrity was such an insult that a man would duel to defend his own honor or to uphold the honor of a virtuous woman. The Blessed Virgin Mary’s testimony to the circumstances of Christ’s birth would have been accepted by all contemporaries of good will on the merits of her transparent saintliness.
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