How do we know Mary was a virgin if she was married to Joseph? He was going to divorce her when he found out she was pregnant. But how did he know that the baby wasn't his?

The gospel accounts of Matthew and Luke both give evidence as to how we know Mary was a virgin and that the baby was conceived of the Holy Spirit, not by Joseph. Matthew 1:18 states that Mary and Joseph are betrothed, thus, not yet married or physically intimate as husband and wife. A betrothal was a binding contract similar to how we consider engagement today, but it was not marriage. Because it was a binding contract, breaking it required a divorce or release of contract. However, we shouldn't confuse this divorce to mean Mary and Joseph were already married. Matthew 1:18 specifically tells us Mary was found to be pregnant prior to any physically intimate relationship that is found within a marriage. Matthew 1:20 tells us that an angel of the Lord appeared to Joseph to tell him that the baby was not from another man, but rather, from the Holy Spirit. Matthew 1:25 tells us that although they did get married, they were not physically intimate until after Mary gave birth to her son Jesus. Matthew 1:23 is a quotation from Isaiah 7:14 - “Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son and will call him Immanuel” (ESV). We are told everything took place in order to fulfill this prophecy, which is another clear indication that Mary was a virgin and that Jesus was conceived of the Holy Spirit.

Looking at the gospel of Luke, we are given more information about the conception of Jesus (specifically relating to Mary). Mary was also visited by an angel, Gabriel, and in Luke 1:31, Gabriel told Mary she would conceive and bear a son. In verse 34, Mary asks how this can be since she is a virgin. The Amplified Bible translates verse 34 as, “Mary said to the angel, ‘how will this be, since I am a virgin and have no intimacy with any man?’" Gabriel replies in verse 35, telling Mary the Holy Spirit will come upon her with the power of the Most High. Again, the Scripture clearly shows that Mary was a virgin and that Jesus was conceived by the Holy Spirit.