The strongest evidence that Marin is Hispanic is that she is from Puerto Rico and may return there. The opening lines of the vignette include, "Marin's boyfriend is in Puerto Rico. She shows us his letters and makes us promise not to tell anybody they're getting married when she goes back to P.R." (Cisneros 25). The "going back" implies that she's either Puerto Rican or spent time there previously.
Other evidence connects a bit more to stereotypes. Young Hispanic women are often seen as fashion and image conscious, and Esperanza describes Marin as wanting to work downtown where you "always get to look beautiful and get to wear nice clothes" and "knowing what cream is best for taking off mustache hair" (Cisneros 26).
Music and dancing also play large roles in Hispanic culture, and Marin is "there every night with the radio... under the streetlight, dancing by herself" (Cisneros 26-27). Finally, Marin is a name with Latin American roots (like Marin county in San Francisco)... the attached link includes a number of other examples of actual Marins.