I'd have to say by very long term experience (years and years of it).. without that, one can overdo things without any signs at all one has over done and then be suffering the next day. I know due to how Ive been all the past couple of months, what I can expect to do today. Ive learnt what things can throw curve balls at all (eg combinations of certain things going on).
eg i currently know that if i go over doing 3 hrs of on my feet stuff per day.. Im very likely to have symptoms the next day. So i try to keep my activity levels well below that, that also then allows me to have "reserve" in case something does go wrong. Im fairly safe sticking to 1.5-2 hrs of broken up physical activity per day (as long as it wasnt hard activity eg gardening). By physical activity.. I mean housework, showering, dressing, cooking etc.
Finding out I had POTS and learning to deal with the POTS, has me completely different symptoms wise to what i was before. Over doing things (when the POTS is going on).. will be shown to me by head pressure building up along with increasing tiredness and loss of mental concentration .. (L'engle make sure you dont have POTS, building up head pressure can be POTS sign of overdoing it). .. this head pressure if further i pushed, is followed by dizziness
With POTS being controlled... one of my warning signals that i could be crashing next day CFS wise if i continued... is a very slightly sore throat. I know to stop all activities if that symptom happens to come in or else i risk relapse...
occassionally overdoing it may manifest for me as a general unwellness..malaise feel.. i feel like im starting to be poisoned and will sometimes get a feeling of like something starting to run down the back of my throat (my throat often goes pusy when that occurs and the back of my tongue goes white), this feel for me can be with or without tiredness. That also is my a sign that i need to immediately stop all activities for that day.
I rarely thou have those things occur, as Im careful to stick to what I know are my limits by keeping a good eye on the clock and watching for how long im being active. I will always check the time before I start an activity and have it planned out when I will stop, even before I start it. I know with any easy physical activity going over 45 mins in one go.. I may be starting to go into the zone where I could send my health downhill a bit. Hence i prefer to break activities down into 30 min time slots. (playing it much safer than I need to do.. the end result thou is the same at 1.5 to 2 hrs of physical activity per day).
When i was more ill.. i'd break it down to 10-15 min time slots with a physical activity level of 1 to 1.5 hrs per day.
it's unlikely you'll get good enough to stop all PEM - but you should be able to stop almost all (but not all) moderate or higher crashes after a while
This isnt true...some of us get to the point of knowing our bodies so well that we can stop all PEM. I have been able to go a couple of months at times with NO SYMPTOMS at all.. even minor ones, just be restricting what i do and keeping within the known limits I have. Where Im usually thrown back into a symptom is when another has made me push out of the limits I like to impose on myself to stop all symptoms. By no symptoms.. i mean NONE at all (except my memory isnt as good as before CFS).. if i havent done anything to cause anyway.
Take today for example.. i feel great. Not tired at all, No CFS symptoms at all (im not counting my current arm injury which wont heal probably having CFS keeps inflammation in my body). If my POTS is controlled.. my CFS symptoms only come in on exertion as I've recovered to a point in which I dont have them all the time and stop them coming in by limiting what i do.
On the rare occassions i do overdo.. I only overdo enough to give me extremely minor crash..
(keeping the CFS symptoms this low to the point of often having none (except memory issues and BP dysregulation issues) is no easy feat.. as I did have severe CFS. It can be done (once one is to the point in which ones daily completely necessary stuff doesnt make one any worst, till one gets to that point thou, it cant be. This thou may be impossible for ones with CFS/ME who have got responsiblities eg young children to look after).