Cunningham tribe—one branch, that is—was gained from events of last winter... We watched. One morningJem and I found a load of stovewood in the back yard. Later, a sack of hickory nuts appeared on the back steps. With Christmas came a crate of smilax and holly. That spring when we found a crokersack full of turnip greens, Atticus said Mr. Cunningham had more than paid him. (Lee, 21)

Another flashback scene takes place in chapter 12 before Calpurnia takes Jem and Scout with her to First Purchase African M. E. Church. Scout recalls her previous experience when she and Jem were allowed to go to church unattended. Scout goes on to say,

Calpurnia evidently remembered a rainy Sunday when we were both fatherless and teacherless. Left to its own devices, the class tied Eunice Ann Simpson to a chair and placed her in the furnace room. We forgot her, trooped upstairs to church, and were listening quietly to the sermon when a dreadful banging issued from the radiator pipes, persisting until someone investigated and brought forth Eunice Ann saying she didn’t want to play Shadrach any more—Jem Finch said she wouldn’t get burnt if she had enough faith, but it was hot down there. (Lee, 118)

Before Bob Ewell takes the witness stand in chapter 17, Scout utilizes a flashback for the reader to gain background information on the despicable Ewell family. Scout remembers her experience last Christmas when she saw the Ewell household for the first time. Scout says,

Atticus took us with him last Christmas when he complied with the mayor’s request. A dirt road ran from the highway past the dump, down to a small Negro settlement some five hundred yards beyond the Ewells'... In the frosty December dusk, their cabins looked neat and snug with pale blue smoke rising from the chimneys and doorways glowing amber from the fires inside. There were delicious smells about: chicken, bacon frying crisp as the twilight air. (Lee, 173)

Overall, Scout utilizes flashbacks anytime she breaks from the chronological sequence of the narrative to describe a past event, which gives the audience relevant background information to the story.

This is a bit difficult considering the entire novel is told in retrospect, that is, as one united flashback. The book as a whole, in this case, could be called a fictional memoir, or a memoir written as creative nonfiction (since it is loosely based on Lee's childhood). This all has more to do with the genre. Flashbacks can occur in fiction or nonfiction. 

More to the point of the question, what classifies as a flashback in a novel told completely in retrospect? One way to answer this is to look for flashbacks within the time frame of the novel - within Scout's childhood. So, any moment when Scout goes to the past (namely, within the time frame of her own childhood) would classify as a flashback. 

In Chapter 2, Miss Caroline reprimands Scout for learning to read from Atticus. She then tries to recall when and how she learned to read. This is a brief flashback: 

In the long hours of church—was it then I learned? I could not remember not being able to read hymns. Now that I was compelled to think about it, reading was something that just came to me, as learning to fasten the seat of my union suit without looking around, or achieving two bows from a snarl of shoelaces. I could not remember when the lines above Atticus’s moving finger separated into words, but I had stared at them all the evenings in my memory . . . 

Perhaps the most significant flashback (as it pertains to the title and theme of the novel) occurs in Chapter 9. 

When he gave us our air-rifles Atticus wouldn’t teach us to shoot. Uncle Jack instructed us in the rudiments thereof; he said Atticus wasn’t interested in guns. Atticus said to Jem one day, “I’d rather you shot at tin cans in the back yard, but I know you’ll go after birds. Shoot all the bluejays you want, if you can hit ‘em, but remember it’s a sin to kill a mockingbird.” 

This flashback then returns to the present with Scout asking Miss Maudie about why it is a sin to kill a mockingbird.