My first attempt was
Indeed he (knows not) how to (know who (knows not also (how to unknow)))
Meaning
This person does not know how to distinguish between those who know how to unknow, and those that cannot.
Now my brain doesn't recognize "know" as a word ...
Ambiguity-resolving trick: if phrases can be interpreted as parallel, they probably are.
Recognizing that "knows not how to know" parallels with "knows not also how to unknow," or more simply "how to know" || "how to unknow", makes the aphorism much easier to parse.
Rules: