I have been advised by doctors to consider CFS/ME but I can’t tell if I fit the criteria

My main discrepancy is regarding PEM. I have in the past been able to push myself and go to the gym on a good day and not crash afterwards. In this case I walked on an inclined treadmill, in an attempt to improve my cardio. The next day I felt fine. But I dont believe I kept track of how I felt after that following day. I have since had a few crashes following things like trying to go to a golfing range, leaving after 20 minutes of what felt like intense exercise and being stuck on the couch for 6 hours after it. In my past year of intense fatigue, it certainly has varied but on a CFS scale it would be in the mild range. I have ruled out nearly every other cause, I even started CPAP therapy for moderate OSA. Since the beginning I have been told exercise is the way out of my situation, although I have instinctively avoided anything crazy simply because I didn’t want to hurt my body. I am posting here because it is crucial that I determine if I have CFS, and the most important part of that is being able to identify PEM. I have no idea how to do that, especially because after a year of low activity my body is so deconditioned it would be hard to differentiate PEM from DOMS. The extent of my current activity level is 30-45 min walks during my good days.