First of all, I understand that these typological distinctions are not absolute and almost all languages show signs of almost all morphological strategies but most display a certain tendency towards one or the other.
I also understand that agglutinative languages are said to use bound morphemes called affixes for many grammatical functions where isolating/analytic languages prefer independent function words like adpositions and particles.
What I'm not sure about is how an affix is differentiated from a particle. As a result, I'm not sure about how to distinguish (mostly) agglutinative languages from (mostly) analytic languages.
I'm a native Turkish speaker. In Turkish, affixes undergo certain (mostly predictable) phonological assimilations (like vowel harmony) according to the base morpheme they're attached to. Less often the base morpheme also undergoes similar (again, mostly predictable) changes. This is how I differentiate, say, a case suffix from a postposition. The former undergoes the assimilations while the latter doesn't. For example:
yaprak + den = yapraktan ~ from (the) leaf
yaprak + gibi = yaprak gibi ~ like (a) leaf
Here, I can tell that the first one is a case suffix because it undergoes vowel harmony and voice assimilation while the second is a mere postposition because it doesn't undergo similar changes.
Are these criteria applicable to other agglutinative languages? If not, what other methods are used to distinguish an affix from an independent function word?