She knew because she could see his jaw working.

Kaz turned away abruptly, but not before she saw his jaw tighten.

Odo's eyes were good enough to see the captain's tense jaw and shoulder muscles.

She saw Lloyd's jaw set in a tight line.

Andy looks from him down to me; I can see his jaw trembling.

I saw Allan's jaw set in a tense line as we followed.

She saw his jaw tighten, and the dark gleam which seemed to come into his eyes.

You could see his jaw now and his fine strong teeth; he must have been young.

Bart could see Jamie's jaw moving as if he were trying to form some words, but nothing was coming out.

From time to time, Janeway could see his jaw tensing.