How would you go about determining where you’ve already been in 1.20 caves?

I use a pretty simple method, not only for 1.20 caves but for any branched exploration:

• When you come to a crossroads or branching path, put a single torch down on the floor in the direction you came from.
  • This creates breadcrumbs you can follow to retrace your steps.
• If you come to a dead end, backtrack to the last crossroads, and put 2 torches down next to each other at the entrance to the path that was the dead end.

When backtracking, if all of the paths are double-torched, you've explored the entire branch. Take the single torch path (the way you came from), and put a double-torch down in that direction at the next crossroads, to mark the whole branch as explored. If you're short on torches, you can pick up all of the double torches before following the single torch path, since you won't be going back down that way again.

When placing torches to light a path normally, use the walls, not the floor, to avoid confusion. If you're worried about your ability to remember this, you could use redstone torches for the markers instead of normal ones.

Torches on the left, so right to the light puts the entrance in sight.

Anything on the ground itself is for ambient light only

How I do it is torches on the left to know the right way home, and any dead ends I'll block with a horizontal line of cobblestone, or maybe just 2 cobblestone on either side of the entrance if I'm running out.

I make arrows out of diorite or granite since I keep picking it up while mining.

I put torches down as I explore.