Compounds and Phrases [compound nouns vs free combinations, collocations]

First, terminology needs to be defined. Here, 'word' (orthographic word) and 'phrase' (meaningfully constructed string not containing finite verb and consisting of two or more words) are used:

Giegerich argues extensively that

steel bridge and watch-maker are unequivocally phrasal and lexical respectively.

[ie a phrase and a (compound) word respectively]

The analysis really boils down to 'is XY better considered/treated as a coherent unit or as a closely-related pair of units?'

He adds

Establishing [these] two prototypes will facilitate a more informed subsequent discussion of the less clear-cut and possibly borderline cases ...

He spends pages attempting to establish the truth of his assertions. And it's a difficult process. And these are not two of 'the less clear-cut and possibly borderline cases'.

I feel entitled to quote his 'Linguists continue to argue' slightly out of context.

My advice is not to worry too much about when a string has graduated from free combination (ie 'casual acquaintance') to strong collocation to open compound. Use them the way most people do, and be aware that the analysis is very tricky and open to dispute.

Telling the difference between a compound noun and a modified noun is not straightforward, but one way that often works is to consider the stress. A modified noun has primary stress at the end, while a compound noun has stress at the beginning. There are exceptions, but it works for your examples, using 2 to mark secondary stress and 1 for primary stress: "2steel 1bridge" (the modified noun), "1watch 2maker" (the compound).