True, I'm sure some others could gain something useful, but a health forum would probably be a better place for all the details. I think there's a small LessWrong walk away message though, so I'll elaborate a bit here.
I began having various symptoms about 5 years ago. The main things were heart issues (tachycardia, brachycardia, arrhythmias), digestive issues, low body temperature, and cognitive issues (confusion, reduced reflexes, poor multitasking). I was a typical junk food-eating,over caffeinated college student at the onset.
After about a year of worsening symptoms I sought out medical help. Multiple doctors had nothing to say. The last doc I saw diagnosed me with hypothermia. I slapped my face and decided I could do better on my own, and thus started my food experiments. It took a few years and was more difficult than I would have thought (being skinny I never realized how dependent I was on certain foods) but I finally converged on a diet that has largely eliminated all my symptoms.
Treating food like an experiment, rather than a source of pleasure or a chore, has been a great way to exercise rationality and will power. Nutrition is a complicated subject, confounded by conflicting information and individual differences. In addition there are numerous motivations for rationalizations (social pressure, convenience, cravings), and often little confidence about rational conclusions. Combine all that with a need to make level headed hypotheses and the ability to carry out the month long experiments and you have yourself a tricky exercise.
We all have our good days and our bad days. Due to insufficient sleep, illness, stress, distractions, and many other causes we often find ourselves far below our usual levels of mental performance. When we find ourselves in such a state, it's not really worth putting effort in doing many tasks, like programming or long term planning - as quality will suffer a lot.
The problem is - other than observing deterioration of results, I have no idea if I'm in such a state or not. I cannot be sure if it's also true for others, but I had to find out a few tests of what's my mental performance at the moment. Tests that are deeply flawed, so I'd request better if there are any. I also cannot predict my mental state in advance, as my life isn't terribly regular.
The most reliable test I found, and by accident, was fighting bots on a certain Quake 3 map - me vs 10 or so highest difficulty bots. The challenge was to get 50 frags without dying. As the map was huge and full of power ups, it wasn't really that difficult as long as I could maintain full alertness for 10-15 minutes - but if I was tired or distracted, I would invariably fail. This test was unfortunately extremely slow.
Another test would be to go to goproblems, and do a few random problems at proper difficulty level. If I could think right, I would do most of them, if I was tired, I would fail almost 100%. This didn't test alertness, I guess it would be best described as short term memory test, as that's what used for game tree exploration. Unfortunately what's the proper difficulty varies a lot with how much go I played recently, so it needs to be recalibrated.
One more test would be to go to some decent online IQ test like this one. My results on such test would suffer a lot if I was sleepy or tired. The main problem is that such tests cannot repeated too often, or I'd just remember the answers.
So these are three ways to test how well my mind functions at the moment, all testing something different, and all flawed in one way or another.
How do you test yourself?