And this has nothing to do with “show, don’t tell” or “avoid passive voice” and subjective things like that.
This is the best advice I have ever heard, and it came from Maxwell Alexander Drake, who is a reputable author, and who also gives free advice at https://www.drakeu.com/
For those of us lucky enough to live in Las Vegas, he does free monthly classes, year-round. And he has several excellent craft books out.
Now I think I’ve said enough to make me feel like I’m paying homage rather than just sharing this genius tip without his permission:
Go through your completed book, and highlight all the dialogue. Choose a different color for each recurring character. Then, you can scan through quickly, reading all of one person’s lines all at once, to make sure they sound like they’re in character throughout.