There is not definitive way to know. Poor sleep, depression, and other things can be the cause, too.

In my case, the only indication was when I had much better days cognitively when I slept with better posture.

Like so many issues with concussions, trying different treatments or behaviors for an extended period can effect improvements.

There are no direct diagnose and treat protocols. In much of medicine, the try this and see what happens protocol is a mainstay.