This is an excellent question. If you think of a poem as “an imitation of an action by language” (Aristotle’s definition), and if you remember that “verse” has only one narrator (prose has more than one; drama has none), you can begin to get at the “theme” by determining the narrator’s voice and motive for speaking (sometimes called the “speech act” of the narrator). Then take a close look at the setting (technically, the “mise-en-scene”)—the physical place, the time of day and year, etc. Finally, look at the poet’s reason for writing the poem; what is her or his rhetorical attitude toward uttering these words? What metaphors and other figures of speech reveal the poet's "stance"? The theme, then, is ready to be named when you “abstract” these features – Love for a woman? No, but "how her beauty compares to the beauties of Nature"? Yes. Flowers are pretty? No, but “The vastness of the universe can be perceived in the details of Nature”? Yes. Comparing and contrasting themes, then, becomes an act of penetrating the poem’s surface to reveal the larger, higher purpose behind the poet’s putting pen to paper.