It is common knowledge that na-adjectives are in truth nouns that can be used as adjectives (edit: I now see this was a false generalization I was led into believing by some grammar sources); and the commonest formula where it's obvious the na-adjective is used as an adjective is "na-adjective+な+noun". But when they are used as the predicate in noun sentences (i.e. "na-adjective+だ/です") I can never be sure if what is meant is a noun, or an adjective.
For example, below are examples where a na-adjective is used as a predicate:
これは幸せです。-> Does this mean "This is happiness" OR "This is happy (this is a happy occassion)
空はきれいです。-> Does this mean "The sky is beautiful" OR "The sky is beauty (the sky is the definition of beauty)"?
夜は静かです。-> Does this mean "The night is silent" OR "The night is silence (in a poetic sense)"
While in the above examples, the na-adjectives are grammatically nouns; most of the time in the English translation, they are adjectives. I'm assuming the first translations I've given above would be the commonest choice of meaning. However, in the case of 幸せ, what is meant is a noun; in the case of きれい and 静か, what is meant is an adjective. How does one tell if a na-adjective used as a predicate is meant to be an adjective and not a noun, or vice versa?
Is it simply a matter of how often they are used as nouns when used without な? (i.e. do we translate 幸せです as "this is happinnes" as opposed to "this is happy", simply because 幸せ is very often used as a noun?)