1. I did not receive any e-mail from friends or colleagues after 8pm last night. Usually I get at least something from someone. This causes me to suspect that e-mail is down.
2. My methods students had a weekly assignment due via e-mail at midnight last night, although they are allowed to skip any two weeks' assignments over the course of the semester. I only received assignments from less than half the students, and, more pertinently, I did not get any assignments sent after 8pm last night. This causes me to strongly suspect that e-mail is down.
3. I look in my spam folder, and I see that I did get a single spam message after 8pm last night. So now I know that e-mail must be down.