I think Peat's mentioned a few things it's helpful to look for.
One would be body temperature, since a cold brain is a slow brain. But body temperature may be hard to measure reliably, and so another approach is to raise body temperature gradually with heat (especially very bright redlight) until you're perspiring copiously. Peat's notion is that as the body warms, the brain's activity should, too.
A second thing to check for is reaction times on a computer. http://www.humanbenchmark.com/tests/reactiontime
If you had a bad night's sleep, your reaction times will be slower than usual, and you'll be likely to feel brain foggy.
One more thing is to prick your finger and check your blood sugar levels with a meter. If you see your blood glucose is low, then you're likely to have brain fog that can be relieved with orange juice.
Finally, how is your gut production? If you're straining, and there's too much bulk, you might want to try eliminating fiber and starch from your diet and using quinones (like cascara) and anti-microbials.
The gut-brain connection may be profound, and Peat describes how serotonin and endotoxins play a crucial role.