hmmm..that's an interesting question...what does it look like to someone else, and what does it feel like tot he person with PTSD? I remember being very surprised the first time someone said to me that I had been "triggered". I had called a crisis line while attending an off campus course at which I was notallowed to do any self-care, and the crisis line person based his assessment on the sounds I was making (ie crying while talking, breathing hard, trouble saying much).
From my side, there is usually a clear progression (now that I have learned to recognize it) but the sequence can speed up depending on how stressed out I already am when it starts. It's something like this: first: basic fight or flight responses --dilation of pupils, increase in heart and breathing rates-- next, narrowing of focus --eg can hear the professor lecture but can't understand the words, concentrating too hard on every individual in the crowded room and all the openings into it that could harbor danger-- finally, physical action appropriate to the situation. That really just means either a) smashing keyboards and the like if I am at home or b) getting up and running away if I am anywhere else, including literally going over the top of or through anyone or anything in the way of the most direct or safest way out, and continuing to run until I have reached a "safe place". From what I have heard, from the outside this looks like : first: scared of something next, either "spooked" or "daydreaming/not paying attention".
I have friends who expect me to just say "I need to get out of here" and then GO when whatever it is starts bothering me. They either follow moire slowly or simply come and find me in a few minutes, because they know I will stop as soon as I can feel safe enough to do so, and that will be someplace I can monitor the exit they need to use if possible and if not possible they know what sort of areas to look for me in.