You've got to keep eye contact, and Snape wasn't blinking.
6 seconds sound clip from the Harry Potter and the Sorcerer's Stone movie soundboard.
You can hear this line at 01:28:14 in the Blu-ray version of the movie.
Quote context
[...]
- Then I lent him to Dumbledore to guard the...
- Yes?
- But Hagrid, whatever Fluffy's guarding, Snape's trying to steal it.
- Codswallop. Professor Snape is a Hogwarts teacher.
- Hogwarts teacher or not, I know a spell when I see one. I've read all about them.
- You've got to keep eye contact, and Snape wasn't blinking.
- Exactly.
- Now, you listen to me, all three of you.
- You're meddling in things that ought not to be meddled in. It's dangerous.
- What that dog is guarding is strictly between Prof. Dumbledore and Nicholas Flamel.
[...]