Recently I have been researching the topic of nominalizations. I learned that such structures might be created by means of morphological derivation (be it affixes, clitics, light verbs) or zero-derivation. I also learned that a term "conversion" lies somewhere very close to zero-derivation. I have been struggling with understanding what is conversion ever since. I mean, technically it is pretty straightforward, things just change syntactic categories, however, I have been wondering how do we know that for example it is a verb that turns into noun, and not the other way round?
For example in a language like Mandarin Chinese there are very many cases of words that switch categories, such as 死亡 (N death, V to die). How to determine in which direction does the category changing process work? What kind of tests could possibly be conducted in order to find out?