How to determine if a pre-head dependent of a noun is a complement or a modifier

These examples are from CGEL*.

a linguistics student

a first-year student

CGEL says 'linguistics' is a complement of the noun 'student', whereas 'first-year' is a modifier of the noun 'student'.

How exactly do you determine the former is a complement and the latter is a modifier?

Also, in the following examples of my own choosing, are the words in bold complements or modifiers of the respective subsequent nouns? And how do you reach your conclusion?

a college student

a police station

a tax bill

*The Cambridge Grammar of the English Language by Pullum and Huddleston (p439)