If they exist, you can find similes in the poem (or in any piece of text) by skimming for the words "like" or "as," or looking for places where the speaker is comparing one thing to another thing.
"Mending Wall," however, is certainly not brimming with similes like many other poems are. The speaker of this poem is very matter-of-fact, very realistic, and he describes images and actions as they truly are.
However, if we look toward the very end of the poem, we'll find one definite simile and one comparison that we might also label a simile:
"I see him there
Bringing a stone grasped firmly by the top
In each hand, like an old-stone savage armed."
Above, the speaker notices that his neighbor is working on the wall by holding tightly to the top of a stone with each hand. He compares his neighbor to a savage, perhaps a caveman, who also grasps a stone and uses that as a tool or a weapon. The simile between the real neighbor and the imagined savage expresses the speaker's slight distaste for his neighbor.
We move a little farther down the poem and notice this observation, too:
"He moves in darkness as it seems to me,
Not of woods only and the shade of trees."
Is this a simile? It has the word "as," right? And it's also a comparison between what's real and what the speaker imagines. But whether we call it a simile or not depends on how strictly you define the term. We could say, yes, this is a simile between the neighbor working in the shade and the savage working in darkness. Or we could say, no, this is simply an example of exaggeration or general figurative language.