How do you tell if a mouse is afraid?

OK, I know that sounds like the setup for a really bad joke, but it's not.

As I'm browsing medical journals looking for data for a research paper I'm doing for my psychology class, I come across the following article: "Systemic blockade of D2-like dopamine receptors facilitates extinction of conditioned fear in mice."

I immediately think, So, how do they determine if the mouse is afraid? Look to see if he covers his eyes with his little paws?

OK, so it actually was the setup for a really bad joke after all. Sue me.

I seriously have to not do any research after 10pm. Things tend to get weird when I do.