I don't think there is any real indication that one is coming, and they are of course unique to the individual. In fact, the times that I have one are typically when I start to really feel the stress of unpleasant thoughts and memeories, and then talk face to face with someone about what's troubling me.
For example, one that I had a week or so ago involved me realizing that my anxiety was being triggered by flashbacks. I started describing to my partner how a particular image made me feel completely terrified, and as I was trying to describe it out loud I realized what was happening. My de>
Guilt and shame have never done any of us any good at all.