Omen III: The Final Conflict

Following the grisly suicide of the U.S. Ambassador to Great Britain (Robert Arden), 32-year-old international conglomerate CEO Damien Thorn (Sam Neill) is appointed in his place, an office his adoptive father Robert Thorn once held. Having fully embraced his unholy lineage and run his company for seven years, Damien now attempts to reshape his destiny by halting the Second Coming of Christ. However, Father DeCarlo (Rossano Brazzi), a priest from the Subiaco monastery where Father Spiletto spent his final days and observed Damien from afar since his adopted father's death, acquires the Seven Daggers of Megiddo that were dug out of the ruins of the Thorn Museum in Chicago. Joined by six other priests, DeCarlo plans to kill Damien while finding the Christ Child. Meanwhile, Damien becomes romantically involved with journalist Kate Reynolds (Lisa Harrow). Learning of his assassins and taking out all but DeCarlo over time, he proceeds to mold Reynolds's young son Peter (Barnaby Holm) into a disciple by playing on the boy's desire for a father figure.
After the alignment of stars in the Cassiopeia constellation on March 24, 1982, generating what is described as a second Star of Bethlehem, Damien realizes it is a sign of the Second Coming and orders his followers to kill all boys born in England on the morning of March 24, 1982 to prevent the Christ Child's return to power. A week after a string of thirty one infant deaths, Reynolds encounters DeCarlo as he reveals Damien's true identity to her while giving her evidence of the murders. The next day, Damien rapes and sodomises Reynolds. The following morning Reynolds discovers Damien's birthmark. Damien tells Peter to follow DeCarlo, resulting in Damien learning that his advisor, Harvey Dean, had concealed the date of his son's birth when Peter reports DeCarlo visiting Dean's wife Barbara and revealing her husband's role in the infant murders. Dean refuses to kill his son and makes preparations to flee the country, only to return home and be killed by his wife, Barbara, who has fallen under Damien's control and murdered their child.
DeCarlo later visits Reynolds and reveals that Peter is now under Damien's influence and that the real Christ Child is now beyond Damien's reach. Agreeing to help DeCarlo, Reynolds tricks Damien with the promise to bring him to the church ruins where the Christ Child is in exchange for Peter. The plan backfires when Damien spots DeCarlo first and uses Peter as a human shield against the dagger. As Peter dies in his mother's arms, Damien throttles Father DeCarlo before calling out for Christ to appear before him and "face him". This leaves Damien open to be stabbed in the back by Reynolds using DeCarlo's Megiddo dagger. As Damien staggers through the courtyard and collapses, a vision of Christ appears in the archway above him. Damien scolds Christ for thinking he has won, and then dies. DeCarlo reappears carrying Peter's body and hands him to a praying Kate before they leave the ruins.
Revelation chapter 21, verse 4, is seen, indicating that when Christ returns to earth, peace will reign for all who faithfully awaited the Lord's return.

Question: What does Reynolds discover on Damien?
Answer: His birthmark