*Since you did not specify a certain poem, let me address how to determine tone in poetry in general.
Tone is the author's attitude toward the topic or subject which he or she is writing about. One good strategy to uncovering the author or poet's tone is to take a closer look at the diction provided in the story or poem.
Diction is the author's word choice. Pay attention to any strong or 'stand-out' words used by the writer. Are there any connotations for the word used that come to mind? Consider whether or not the words that the author uses are more positive or more negative. For example, if the poet was describing some ancient culture, it would make a huge difference on whether he used the word 'primitive' or 'barbaric.' The word 'primitive' comes across to the reader as an adjective meaning 'undeveloped or early' which feels neutral to the reader, but the word 'barbaric' would imply that culture is rough, unrestrained, with violent tendencies. 'Barbariic' has very negative connotations. By analyzing the diction, the reader can determine the author's tone.