A simile is defined as a poetic device or figure of speech that illustrates something by comparing it to something else. In literary criticism, the term "metaphor" is used for an implicit comparison that does not use explicit comparative words, and the term "simile" is applied to comparisons that are made explicit by the use of such terms as "like" or "as." As there are no explicit terms of comparison used in the poem "If", the poem does not contain similes. Instead, it uses related forms of figurative speech such as synecdoche, metonymy, and metaphor.
The phrase "your heart and nerve and sinew" is an example of synecdoche, a figure in which the part (in this case various human body parts) are used to stand for the whole person. The phrase "risk it on one turn of pitch-and-toss" is an example of metonymy, where one type of risk, in gambling, is standing in for bold, daring behavior in other aspects of life.