I haven't paid enough attention to this, clearly.
Ironically, I think my best sign of oncoming PEM is feeling good, especially if I'm getting PEM from something physical. If I do enough stuff to get an emotional boost- yippee, I got something done! that is enough to put me in PEM. Any day that I go to bed happy that I have accomplished something, I know I will wake up feeling just awful the next day. It's hard to say how I know it's going away. I never feel normal anymore, just less impaired some days and lately I've been in a constant crash anyway. Can't remember what not crashed feels like, so not sure if I'm just getting worse or just can't dig myself out of the crash, since I do need to eat and bathe every once in a while.
Sure makes it hard to avoid.