Some of it could come back through conversation. Dreams work too.
Flashbacks could work but it depends on the type of memory you're bringing back. If it's something traumatic then flashbacks could occur when seeing a certain tool or weapon or being threatened in a certain way or being touched a certain way, or maybe seeing someone else in the same position they were in. It could work for other types of memories too, but you don't want your character to see a flower on the roadside and then disappear into a memory about their mother for seven pages. I'd keep flashbacks pretty short if you're going to use them at all. (Unless it's a traumatic thing and they "go under" or something. It could work, depending on how you executed it).
Implication could work, too. Or just it could just be said.
Ex. Hannah is walking back from school when she sees it. The flowers are heaped at the bottom of the telephone pole, the picture of the boy faded and blurred with water stains and time. All of the sudden she remembers how Danny died. It's a rush of screeching wheels and the jarring slam of the collision and the awful scraping sound of metal tearing, the glass shattering and blood spattering red, red, red like poppies in the field behind her house.
I hope this helps! (Sorry about the quality of my example XD).
