No, you don't look like you're up to something, but whenever you look like you're not up to anything, you're up to something.
9 seconds sound clip from the The Year Without a Santa Claus movie soundboard.
You can hear this line at 00:10:48.932 in the Blu-ray version of the movie.
Quote context
[...]
- Are you thinking that I'm doing the wrong thing by staying home?
- Of course not, dear. But if you should change your mind...
- Okay, what are you up to, Ma?
- Me? Up to something? Oh, ridiculous. Do I look like I'm up to something?
- Now, it's nothing that need concern you.
- Consarn it, Ma. What's going on?
- They're only trying to help.
[...]