Currently there's no diagnostic assay for CTE. There's no treatment, nor is there any cure, either.
According to Alzheimer's Association, the symptoms of CTE include:
- Memory loss.
- Personality changes (including depression and suicidal thoughts).
- Erratic behavior (including aggression).
- Problems paying attention and organizing thoughts.
- Difficulty with balance and motor skills.
That link goes on to say that there's no evidence that any particular concussion increases the risk of CTE. It could be lots of low intensity trauma, or it might be due to a small number of high intensity traumas. It's still being studied, so we don't know much about it.
At this point in time, you will get no authoritative answers. The only thing you can do is use your intuition. If you're seeing one or more of the symptoms listed above when they didn't exist before, that's something that should make you worried.
Yeah, people close to the person in question are going to be the first ones to notice something. So if you're a fighter, enlist your friends and family. Ask them to tell you if you're acting weird, and list off the symptoms.
There are written and neurological tests that can be taken to evaluate mental function (memory, math, language, common sense, problem solving, speech, vision, hearing, reflexes, etc.). Specifically, tests for Alzheimer's and Dementia. It's not the same as a test specifically for CTE, but a doctor can still use them. You get a score from that, and I suppose the score can be used later on to see if you're going downhill. I don't know if it can be used that way, I'm just speculating. The tests themselves are standardized, so the results shouldn't change when switching to a new doctor.
Hope that helps.