Eliot was writing around the time when folks were all about making poetry sound real. People wanted poetry of the people, by the people, and for the people, so that poetry had to sound like everyday speech. For the most part. The movement had been kicked off a little further back by Walt Whitman and, even a little before him, William Wordsworth, but some might say the crest of the wave really came with the Modernists.
So you can pick this poem out, in a lot of ways, by the nature of the "Speaker" and the way that the poem sounds just like everyday speech. With phrases like "then at dawn we came to a temperate valley" (21) and "all this was a long time ago, I remember, / and I would do it again" (32-33), the poem sounds almost like, well, prose. In fact, there's a lot about this poem's sound that ties it to Eliot's much longer and more complicated piece, "The Love Song of J. Alfred Prufrock". You can check out the "Sound Check" part of that analysis for more on ways to identify classically Eliot-sounding poetry.