oh sheesh does this go into some generative account of semantics?
Syntax? And, no, it doesn't. It's in a theoretical textbook, but that's what you're doing-- theoretical linguistics, based on distributional criteria for word classes. If you don't understand that much (whether or not you know/accept the rest of the theory), then you cannot possibly answer the question you're asking.
to be honest, i'm kind of troubled that no one on this forum can give me necessary and sufficient conditions to determine if there are a category of adjectives in a language. Nothing personal guys, but I didn't think it was a tough question. You can ask anything about the language and I can provide working translations... think of it as a fieldwork exercise!
Translations are far from sufficient for grammatical analysis. If you use translations, you'll find that mysteriously all languages have the same word classes as English! Further, if you rely at all on semantics, you'll also run into major problems ("destroy" vs. "destruction" for example). This is covered in an introductory textbook like Carnie's (among dozens of others).
Or is the training these days so much more theoretically-orientated?
What do you think you're doing? Where do classification criteria come from?
In the end, here's where we are: you aren't using a theory, but you want to know what the arbitrary words "noun" and "adjective" mean enough to determine whether a certain language
has those classes. How can that be determined except by theoretical criteria?
Regardless, a simple answer has been provided: word classes are distinct if there are situations in which a word from one class cannot be used in place a word from another class. For example, prepositions and conjunctions are distinct in English:
I walked in the store.
*I walked and the store.
(That should be obvious, but also a clear explanation of why.)
Now you just need to look at the language(s) you're working with and see whether nouns and adjectives (based on an initial, rough approximation from semantics) are interchangable in all cases. Are there times where only an adjective can be used?
There is no better answer than that, unless you're working in a certain theory (in which case that will still apply, so you may as well rely on it, given that the theory could be wrong). There aren't universal rules out there for what a noun is or what an adjective is.
And that's the problem: we can't do anything (like come up with N&S conditions) for nouns or adjectives unless we know what they are; to work around this, we can assume that such classes exist and then test whether they're distinct.
Personally, I'd recommend reading Carnie. The first couple chapters aren't especially "theoretical" in the sense you're using the word, and they cover all of this in detail.
Finally, on a related note, you may want to consider whether you're talking about syntactic function or lexical nature. Do these words have classes in the lexicon? Or do they just function one way or another in a sentence? This gets very complicated with affixes like verbalizers and so forth.
In a sense, we can say (not sure how valuable this is) that all languages have nouns and adjectives-- things, and words that describe those things. That's on the surface. But whether a word like "blue" is technically two words (blue.ADJ and blue.N) or just one word (blue.N/ADJ) is a major question that depends on how you analyze it.
What appears to be the case is that there are roots and those roots get used in various positions with affixes. This suggests that there is a lot of morphology and possibly no/few distinctions in the lexicon for those roots. But then you get into complicated questions of whether these are grammatical or derivational morphemes and so forth.
