In the book series The Stormlight Archive, there is a character whose intelligence changes every day. He takes a test every morning to determine his capabilities for that day. As the ruler of a kingdom, he's not allowed to do certain things if he's too stupid, or sometimes even too smart that day.
I think a little test like that would be nice. If you have dementia but can pass a test every once in a while, ok maybe life is still worth living. I figure some days will be better than others. But if you can't figure out some basic things for like a month straight, maybe it's time to check out.