How can we assess our own mental state?

For example, in the morning I always thought I am awake but I can always be more alert by showering. How can my brain tell if it's at a sub-optimal state?

Different from this question which is more asking about the origin of the mental state, this question is asking whether we can truly assess its performance.

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    $\begingroup$ You can device psychophysical tests that can tell you something about, e.g., reaction time, working memory, or IQ at the time you wish to assess 'your brain'? What is it that you are after? A type of test? And what do you mean by ''cognitive state', mental state', 'sub-optimal state', and '[brain] performance'? At this point the question needs more focus and background information. $\endgroup$
    – AliceD
    Oct 31 '20 at 20:01