How do you know?

Inés:
[model pointing at defaced portrait]
Why doesn't that painting have a face? Goya:
Because he is a ghost. Inés:
No, he is not. Goya:
Have you ever seen a ghost? Inés:
No. But I have seen a witch. Goya:
Oh, did you? Inés:
Yes, but she had a face. Goya:
So what did she look like? Inés:
She was... all bent and creepy, and she... [whispers:] Inés:
stank. [makes disparaging sound] Goya:
That's interesting, because the witch that I know, she's... she's young, very lovely, and she smells of jasmine. Inés:
[smiles]
She does? Goya:
She does. And I'm working on her portrait... right now. Inés:
[smiles as it dawns on her what he means]
I'm no witch! Goya:
[chuckles]
How do you know?