adverbial or adjectival prepositional phrase?

Hello everyone! How am I supposed to distinguish whether the prepositional phrase in a sentence functions as an adverb or as an adjective? It confuses me expecially when it comes to a sentence with a formation of this kind: subject verb noun prepositional phrase. In such occasions I find out that sometimes the prepositional phrase functions as an adjective and sometimes as an adverb. So I would like to ask if there is any rule to tell what the phrase functions as or do we just have to rely on the context to figure out what will make more sense? Thanks a lot!