In order to find the theme in a poem, it's best to start by understanding the main idea of the poem and think about what is literally happening in it. In "A Drunk Cadet Had Hit Them," the speaker is mainly discussing the sudden death of a friend, Beck, in a car accident. Themes that you may want to discuss are survival, grief, or the loss of innocence.
Depending on the theme you choose to discuss, start looking at the craft techniques the author uses to convey the bigger message about human nature within the text. Hampton's word choice and language in the poem provides evidence of the themes. You can specifically look at lines such as "I sit like an automaton in the big lounge room / my back teeth snapping against each other, / rocks in my gut" or "Voices travel to me from distant realms: / I hear again the singer’s plea for his wife. / Blurred from too much experience" to analyze themes of survival or grief.
The theme of loss of innocence is also one to consider writing about, and it takes many forms throughout the poem. Lines such as "Fourteen, it’s an age to break your heart. You know / nothing. On the edge of knowing too much" show a loss of innocence in a positive tone. Later in the poem, the speaker's friend Kristin divulges information about her own loss of innocence: "Kristin was raped at times twice a day / or sometimes not for a week."