@Peterש Yes, that's right. Some of the many syntactic tests for nouns-vs-adjectives include: §1 A predicate test with a copula or other linking verb, which most nouns fail but most adjectives pass (e.g.: a simple character > a character is simple VS a wildcard character > a character is ❌ wildcard). §2 A morphology test where you see which of inflecting as a noun or as an adjective produces a grammatical result (e.g.: some characters are wildcards VS some characters are ❌ more wildcard than others). — tchrist ♦ 9 mins ago

@Peter §3 A noun-phrase–ordering test because any noun adjuncts must follow all adjectives in a noun phrase, so if you can swap two modifiers then both are of the same word-class not different ones (e.g.: a new wildcard character VS a ❌ wildcard new character). Other useful syntactic tests for this purpose include a substitution test, a coördination test, and testing for the semantic role. In all these tests most attributive nouns will all either pass or fail whereas most attributive adjectives will do exactly the opposite of that. Together the preponderance of evidence is dispositive. — tchrist ♦ 1 min ago

@Peterש Another syntactic test is an intensifier test: most nouns don't like being modified by intensifiers or adverbs, but most adjectives are fine with that. In the same fashion, most nouns can be modified by an adjective but most adjectives cannot be and attempting to do so yields something ungrammatical. Do you see how this business of using syntactic tests works to figure out what something's word class is? There can be individual exceptions here and there, but the overwhelming results will always skew one way or the other. That's the scientific way to determine the answer. — tchrist ♦ 4 mins ago