It depends! For lengthy flashback sequences, I usually just make it clear through narration that it's a flashback. Sometimes I'll do some cool visual effect to show when the flashback is over:
![](https://forums.tapas.io/uploads/default/original/3X/f/b/fb1e466d2a6625a91e8e1a210ad1ae7d6aa8166d.png)
For single panel flashbacks, I just round the borders:
![](https://forums.tapas.io/uploads/default/original/3X/9/5/95fae75b025cfb715a449807cd8c1ef366735111.png)
And for single lines of dialogue, I just put the relevant character inside of a thought bubble:
![](https://forums.tapas.io/uploads/default/original/3X/0/f/0f45bfbb3485e8d4cbb8d63b56dcc03b0cf586f5.png)