How do you know? why are you so sure?
5 seconds sound clip from the The Kid with the Golden Arm movie soundboard.
You can hear this line at 00:47:46 in the DVD version of the movie.
Quote context
[...]
- Now listen, we'll open the case, take all we can, then run. It's better than dying.
- Hey. Now don't touch a thing.
- It's the damn chest. They are being poisoned.
- How do you know? why are you so sure?
- Strange. I just can't see how the gang could have arranged that.
[...]