The hallmark of CFS is post-exertional malaise, which is feeling terribly after exertion that is overly strenuous (for you). The onset of PEM can be a few hours, a day, or even a week or more after the exertion, so it can be very hard to pin down. The real way to tell is to cease exertions for a period and see if there is improvement in your symptoms. Contrast this with depression, where exercise will tend to improve your symptoms.

A second hallmark is unrefreshing sleep. Not necessarily that you don't sleep or don't sleep well, but that no matter how long or how well you sleep, you wake up feeling like you just ran a marathon. If you don't sleep well, see a sleep doctor. That may fix the issue.