You can also gauge how well you solve chess puzzles. A great place for this is Lichess[0], where you are presented with situations from real games played on the platform, and you are trying to find the best series of moves for one side, playing against the computer. Those puzzles have difficulty ratings based on the ratings of players who have been able to solve them or not, and your rating goes up or down depending on if you solve it successfully. It's also great because there's no time commitment or pressure with the puzzles. You can jump in or out at any time.

[0] https://lichess.org/training/

Yeah, I think chess can give you cold hard feedback on how well your mind is working on a given day. As the author already mentioned, performance can vary a lot. When you're doing poorly, you often don't feel very different, but then you sit down to play chess and realize you're basically sleep walking. What you're looking for is a fully awake, high alert, high awareness state of mind. Factors like diet and regular exercise can make a huge difference in my experience.

Absolutely. It is always a really indicator of if I'm tired or not if I start to fail a lot of chess puzzles where I should have seen something, but just didn't notice it because I was tired. As someone who tends to stay up late, it's been nice, because I have an objective measure of my cognitive decline from being tired that I wouldn't have seen otherwise, and it's made me want to go to bed so that I'm not tanking my puzzle rating on Lichess, haha. :)

There's cheaters but the number is so low that it doesn't really make that much of a difference. Also, if they get caught you get your points back.