How do you distinguish prepositional phrase from idioms?

[susumuya]

I find it hard to distinguish the two of them.
Are there any ways to distinguish prepositional phrase from idioms?

How can we categorise idioms into various subtypes?
Like, fall in love (with sb.), run a mile, work out, beside onself, dress up...

If 'keep up sth.' is an idiom, is it still a prepostional phrase?
If yes, why some prepositional phrases are idioms and some are not?

Thanks!!!

[Tdol]

There's debate about prepositional phrases, and it depends on people's interpretation. The Cambridge dictionary of idioms, for instance, takes a wide view and happily includes them. We do too on the site, on the grounds that they might cause comprehension difficulties for learners. With phrasal verbs, some people use the term 'idiomatic', which is one way of avoiding the problem, but it can be difficult to say define things exactly:
The building's on fire.
The player's on fire. (performing well)
Here you can see a difference in meaning, with the second being an idiomatic use. Does that make 'on fire' a prepositional phrase or an idiom?

BTW, I am not sure about the categorisation you refer to.