We can identify modifier nouns because they:
- The modifier noun modifies the prototypical meaning of the head noun.
- The modifier noun is used as a noun generally.
- They cannot be used in predicative position, unlike adjectives, e.g. *the leg is trouser.
Sometimes ambiguous analysis is possible. For example, in plastic spoon, plastic can be analysed as an adjective (the fork is plastic) or as a modifier noun (the fork is made of plastic). Both analyses are equally valid.
This results in a compound noun, which may be compounded into one word, hyphenated, or spelt as two (or more) words.