As someone else with anxiety - I think you'll notice it if you get this feeling for physical reasons. When I got it, it felt different from my normal anxiety. My anxiety was way worse back then, but the "impending death" feeling I got for physical reasons just had a different quality to it. It's hard to describe what the difference was, though.

Hmm... Okay, I think part of the difference was that I also had this weird desperate feeling, just absolutely desperate to be alive for a good long time. Considering the fact that I was also depressed at the time, that was new. I think that actual physical danger often presses the "DESPERATELY TRY TO STAY ALIVE!!!!!!!!!!!!!" button in your brain, and that button feels different from anything else.