I've been reading Janet Burroway's Writing Fiction: A Guide to Narrative Craft for the past while. While she presents heaps of good stuff about writing, I came across some ideas about identifying characters that struck me as really essential (her section on Credibility in Chapter 4: Building Character). She suggests that you identify your characters early to help readers from becoming confused. Burroway says:

...we need to know, preferably in the first paragraph, the character's gender, age, and race or nationality. We need to know something of his or her class, period, and region. A profession...and a marital status help, too.

By getting these elements into a story early (this is not an information dump, but rather a creative description), you can keep your readers from becoming confused as to who is doing what to whom and your story will move quickly.