Updated September 24th, 2021.
Apple’s Mail app indicates which messages you’ve replied to using a tiny curvy arrow icon next to the message, like so:
In the picture above, the red arrows point to tiny little curvy arrows for the messages I’ve replied to. Zoomed in, that icon looks like this:
It’s one thing to know you replied to an email. It’s quite another to know what you said! And, it turns out all you have to do is CLICK THE ARROW and your reply pops right up. Like so:
One little click does it all. Give it a try!
Update: this works in many Mac operating systems, up to and including Big Sur 11.5.2. It does not work in 11.6.