When you are looking for literary devices used in a poem, just noticing them is step one, but connecting them to the meaning or theme of the poem is what is really important.  First you have think about what the poem is about:  four guys in a trench being bombed during a war. 

Some devices to notice:

1.  Repetition of "going to get gassed" and "going to get bombed"  The repetition reinforces the terror of this experience.  There is no escaping the weapons.

2.  Repetition of "They're warning us" and "They're over us" has a similar affect as the above note.  You have to be careful to think about pronoun reference here -- who is "they" in each of the stanzas?  How is the enemy warning these soldiers? 

3.  The strong rhyme and meter make the poem rather "sing-song" which serves as irony in light of the seriousness of the situation, and therefore serves to actually reinforce the how awful this is.

4.  The poet's choice of a first person speaker is important too.  It makes the experience personal, as does the casual/conversational language, such as the slangy, "'Cos" and  the cliche, "Thank your lucky stars."  The personal tone also helps to connect the reader to the speaker of the poem -- perhaps most people would be thinking and feeling the same way in a trench under fire.

Below are some links to a couple of sites that have information on other literary devices and techniques.  You could review these sites, or any other, and look again at your poem for other choices that the poet made in the creation of this poem.