For trial prints I'm making in batches, I use my fingernail to make a notch in the very edge of the print.

A notch in one corner, two corners on in the center of an edge is enough to differentiate pieces of paper for me, even in the dark.
I keep notes on a sheet of paper and reference them to the notches I make. When they are developed, I can look back on my notes and see which is which.

At the end, when I find the exposure/development combination I like I copy my notes to the back of the print using a pencil.