If you detect alarm or fear in someone you’re talking to, STOP! Ask what is causing them alarm. If there is an actual danger, deal with it (run away, call the police, etc.) If there is no obvious external trigger, consider whether you could have done anything that would set off unconscious red flags for them. Reassure them that your intentions are purely friendly. Work with them to understand what you did to trigger that reaction.
Nonlinguistic signs of fear (less intense for milder alarm, more intense for terror) are generally involuntary, physical reactions. (It is seldom to someone’s advantage to deliberately signal that they are afraid, and if they did particularly want you to know, the easiest thing would be to just tell you so.)
There is generally some trembling (lips only and/or very minor trembling for milder alarm, the whole body and more violent trembling for intense fear), and you can hear the quiver in their voice and the increased breathing rate that go along with this. Their mouth may go dry, making the tongue stick to the roof of the mouth (you can hear quiet click sounds made by the tongue releasing as they speak).
If the alarm or fear is due to a sudden, startling source, they may have a “flashbulb” effect, where the eyes go suddenly wide, and they will stare with their pupils enlarged and increased blinking. Their muscles go rigid, giving them a tense expression (though they may have a fear-related smile reflex), and a tense tone of voice. Many people will “freeze” (go very still, cease all movement, be “a deer in the headlights”), but others may squirm (if mildly alarmed) or (in more extreme cases) cry, scream, crouch, attempt to protect their bodies with their arms or appear to be pushing out (as if warding away things that might come at them).
The words tell us only that “that’s my favorite story,” but she sounds and looks alarmed when she says it, as if a stalker has revealed that he knows too much about her.