Usually, though, flashbacks are not put in italics, but merely introduced by dropping into a "more past" tense (e.g. from present to past, or from past to pluperfect**) as an introduction to the flashback, then returning to the normal narrative tense for the rest of the flashback, before a quick visit to the past/pluperfect on exiting the flashback.
The main indication that one is in the flashback (or has returned to the main narrative) is the content of the words, which should let the reader know what's happening (unless you're keeping this mysterious for other purposes).