I find that the easiest way to recognize an adverbial modifier is to ask yourself what kind of question it answers.
For example, in this sentence, the modifier answers the question "for who?" which is a question about the verb. However, the information IN the preposition can also be another part of speech, like an indirect object. It is not an indirect object here, though.
The fact that it is additional information and that the sentence would still make sense without it indicates that it is not an indirect object.
Oh, and the adverbial modifier is a prepositional phrase, marked by the preposition. Everything after the preposition is a noun phrase; the easiest way to identify it in this case is with the article "the."