Lucy Cat, all I can say is that for me, ADD/ADHD thoughts are often fleeting, scattered, unfocused and flooding but emotionally there is no blow back. PTSD thoughts pack an emotional or body reaction, and are tied to memories of traumas, triggers, retrieving repressed memories and produce anxiety, fear, anger/rage, fight/flight/freeze/faint, sometimes it causes compulsive or obsessive thoughts or suicidal ideation.
Early on, I couldn't discern the subtle differences. It takes a lot of presence of mind for me to be able to sort it out, but having had a good deal of practice, it is a pretty spontaneous self check now. It is not though 100% accurate, but is more of a two thirds thing. It is like my physical situation, trying to discern the difference between an allergic flare up, or catching a cold or the flu. I focus though on the emotional impact more though and the types of thoughts I am being flooded with to give me cues about which is which... just like I focus on what I ate, or what my environment has been, or if someone infected has had contact with me to give me clues about my physical state. It is not perfect, but it has helped me to recognize that not all my thoughts destructive. Some are ADD/ADHD, some are PTSD.
Not convinced I answered very well. Best I can do today. I could write an essay about it probably... because there are lots of little tells and cues for me. It is just something that I learned by trial an error.