An Ethics Question for Suspicious Professors

When you teach, you do not always get to know all the names of all your students. Generally I have about 100 new students each semester. This semester I have about 75, and I hope that by the time it is over I will know the names of 50 of them. But despite this, when you walk in to your class, unless it is one of those large lecture hall classes, you will recognize everyone. If someone is there who does not belong, you will notice. If there is a person who has never shown up, certainly by the third week you will know it.

So when you give an exam, it is not likely that someone will try to get their friend to sit in and take the exam for them. If they do, you will know it right away. If you have a large class, or you are particularly untrusting, you can ask your students for a photo ID to prove that the person registered is the person taking the exam.

Now what do you do when you have a female Muslim student in your class who wears a veil that covers her entire face except her eyes. You will never know if it is her, her sister, or her friend in your class at any given time.

Should a professor in that position just let it slide? I would not want my teaching suspicions to interfere with her religious right to wear whatever she wants, I just want to have some way of verifying that I am teaching the same person who I am testing.

Addendum:People have apparently taken advantage of the anonymity of veils for illegal activities before.