A few features from Robert Frost's "A Late Walk" identify it as a poem. The first indication is the four line stanzas. Some poetry is not consistent in stanza lengths, but consistent stanza length is a key feature of a lot of poetry.
A second feature is that the poem contains a rhyme scheme. The scheme is ABCB. What that means is that the 2nd and 4th lines of every stanza rhyme with each other.
The lines of each stanza also alternate between 9 syllables and 6-7 syllables. This rhythmic meter creates a lyric quality to the poem, which is also typical of poetry.
Poetry also tends to have a greater use of alliteration than prose. Alliteration is a repetition of a consonant sound, usually within a single line of poetry. A few examples from the poem are "withered weeds" and "garden ground."