Hmm, I have noticed myself rationalizing "I'm going to play just one game of blitz Go" as a test of my mental performance. I'm pretty sure it's a rationalization, and therefore works the other way round.

I would tend to be suspicious of the above tests on that basis, though I can hardly assume your mind's workings are in any way similar to mine. ;)

It would seem simpler to assume that your causal model (tiredness etc. -> loss of performance) is in general correct, and to organize your work accordingly.

Some time management approaches (e.g. GTD) encourage that sort of self-awareness by explicitly recognizing that some "contexts" (which can include your current self-assessed level of energy if you set your GTD up that way) are more conducive to some kinds of work. This could be understood as a formalization of Structured Procrastination.