How do we know Mary was a virgin at the time of Jesus' conception? Luke 1:26-26:


  26 In the sixth month of Elizabeth’s pregnancy, God sent the angel Gabriel to Nazareth, a town in Galilee, 27 to a virgin pledged to be married to a man named Joseph, a descendant of David. The virgin’s name was Mary.


Many other Bible verses talk about Mary's virginity, even to the point that the Catholic church teaches she was a virgin her entire life*.

What evidence exists (either Biblical or extra-Biblical) to support the claim that she was indeed a virgin at the time of Jesus' conception?

*This question is not asking whether Mary was a perpetual virgin; only for evidence that she was a virgin at the time Jesus was conceived.
Matthew 1:25 (NIV)


  But he did not consummate their marriage until she gave birth to a son. And he gave him the name Jesus.


This shows that she didn't have sex with Joseph until she had given birth.

Furthermore, it fulfilled prophecy:

Matthew 1:22-23 (NIV)


  22 All this took place to fulfill what the Lord had said through the prophet: 23 “The virgin will conceive and give birth to a son, and they will call him Immanuel” (which means “God with us”).


From the prophecy of Isaiah:

Isaiah 7:14 (NIV)


  Therefore the Lord himself will give you a sign: The virgin will conceive and give birth to a son, and will call him Immanuel.


From a different gospel, we have this account:

Luke 1:30-35 (NIV)


  30 But the angel said to her, “Do not be afraid, Mary; you have found favor with God. 31 You will conceive and give birth to a son, and you are to call him Jesus. 32 He will be great and will be called the Son of the Most High. The Lord God will give him the throne of his father David, 33 and he will reign over Jacob’s descendants forever; his kingdom will never end.” 
  
  34 “How will this be,” Mary asked the angel, “since I am a virgin?” 
  
  35 The angel answered, “The Holy Spirit will come on you, and the power of the Most High will overshadow you. So the holy one to be born will be called the Son of God.


If you want, we can get into the Greek and Hebrew here, but it's pretty clear all over that she had not had sex with anyone and yet she became pregnant.