Infinitive Phrase or Verb Constituent

I refer to Doing Grammar by Max Morenberg as my default grammar reference.

Morenberg makes a distinction between infinitive phrases, which function as Noun Subjects & Objects (SC, NDO, etc), and infinitives that are part of a verb phrase:

I want to see the evidence (1 verb phrase, want is a modal)

I agreed to watch the boy. (noun phrase, functioning as Direct Object)
I asked to see the evidence. (noun phrase, functioning as Direct Object)

How do you distinguish between verb phrase constituent and a separate noun phrase?