The speed and depth of breathing, which shifts with calm and with arousal.
As a tense topic arises the breaths become shorter and higher in the chest. The change in breathing arrives before any words, giving the pressure away early.
Quick shallow breathing can signal stress, while slow deep breathing suggests calm. A sudden shift in rhythm often lines up with the moment a subject begins to unsettle someone.
Exertion and health affect breathing, so rule out physical causes first. Someone short of breath from the stairs or from illness is not necessarily anxious.