You can both start being aware of your particular patterns of zoning out or heightened arousal as they happen. The "symptoms" of hyperarousal of the nervous system are fairly similar across human experience, s it shouldn't be too hard for your T (I'm kinda blown away that he doesn't already know how to recognize a trauma response when he sees it). But it is true that everyone does have their own unique set of ingrained responses.
Anyways, if you and he reflect on past experiences of you becoming hyper (or hypo) aroused in session it shouldn't be too hard to identify your particular responses. My own are quickened breath, still body, averted and fixed gaze. Once you know what your particular responses are the onus should be on YOUR T, not you, to notice the physiological response and respond to it.
Also communicate with your T about what responses are helpful for you and which are not.