We must distinguish judging from guessing.

It is possible to produce something that is grammatical either by chance or under the supervision of another. To be proficient in grammar, then, one must both produce what is grammatical and produce it grammatically, that is, in accord with knowledge of grammar in oneself.

The success of an archery shot may bring food to the hunter's starving family, or may constitute a horrible murder. But these outcomes are irrelevant to the assessment of that shot as a hunter-archery shot, as an attempt to hit prey without running excessive risk of failure.

You attain aptness by judging while in good shape and in a good situation (good light, good distance, etc.), through the exercise of good barn-sorting epistemic competence.

Judgment is affirmation with the intention to thereby affirm competently enough, and indeed aptly. That distinguishes judgments from mere guesses.

When you dream, your perceptual (and other) competence is affected. You are then unable to get it right competently with the beliefs in your dream.

Arguably, if you view a real barn in bright sunlight and close by, while fully alert and otherwise in good shape, then you do know whether or not you see a barn. You have "animal" knowledge, says my virtue theory, through the first-order aptness of your judgment.