I think it takes time to recognize when your having a flashback. It takes a lot of mindful awareness and practice.

It is helpful to "check in" with yourself from time to time. Take an inventory how your physical, mental, and emotional state. What is your posture like? Where do you feel tension? Or do your muscles feel relaxed? How are your thoughts? What emotions do you feel and where in your body do you feel them?

The more you become aware of how you feel, the easier it will be to recognize when your either amping up to have a flashback or are in an actual flashback.

Just checking in with yourself can be a challenge. It is for me. Sometimes I'm afraid I won't like what I notice, but once I do it, I feel a sense of relief and satisfaction.

I think it's also important to develop some grounding techniques, like from DBT, so that if you notice something uncomfortable you can use your skills to calm yourself back down. DBT also utilizes containment strategies, which I do not have a lot of experience with. But they can be helpful as a temporary solution when you don't have the energy to face things in the moment.