You just got a call from the University system administrator, who says that either

You just got a call from the University system administrator, who says that either you or your roommate is issuing denial-of-service attacks against other students from your shared network segment. You know you are not doing this, but you are unsure about your roommate. Questions:
1. How can you tell if this accusation is true or not?
2. And if it is true, what should you do about it?