This is something I've worried about ever since I was a teenager. If something happens to my mind, how will I know? The answer I've come up with is that I can't fully know, but my best bet is to "reason on paper".
Externalize the facts and opinions (write them down or say them out loud) and then run logic on them using some explainable algorithm. Basically, do it like we do math: the numbers and figures on paper will allow anyone to continue where you stopped. Even if you forget what you were doing or you can't do large scale abstract reasoning, because you've written it down, you can do it piece by piece.
When I suffered what seemed to have been a "ministroke" (basically a stroke that solves itself) my cognitive abilities were obviously affected. I seemed to operate basically only on short term memory - several seconds at a time. Anything that happened before that seemed to fade from memory. Saying out loud what I wanted to do and then continuously repeating it out loud enabled me to do more complex tasks. Unfortunately, I had no sense of danger from it.
Writing (typing), however, was not really possible. I knew what I wanted to write, but it became "slurred". I hadn't considered that before. I even recognized myself that what I wrote was difficult to decipher, but somehow I just couldn't type out the words correctly.
I still think that reasoning on paper or out loud is your best bet in these situations.