A new dashboard design, including multiple buttons, ...
Dictionaries I've looked in only list "multiple" as an adjective. It makes sense to me that in the above sentence, it could be syntactically an adjective, because I can easily replace it with other adjectives "including green buttons" etc., but I could also replace it with certain determinatives like "two", "some", "these", without it becoming ungrammatical.
Furthermore it seems to tick the boxes as far as I can see of other quantifier determinatives, always seemingly interchangeable with "several" for example, which dictionaries do tell me can function as a determiner.
How can I disambiguate whether an adjectival/determiner-positioned word in a NP is a determiner or an adjective?