Differentiate Value of Unmarked Potentiometers

I'll suggest something wacky, in case you don't want to or can't modify your pots. How about connecting two standard potentiometers together with a mechanism, such that twisting a knob moves one pot, and twisting the knob in the other direction moves the other pot?

One possible way to do this is to have two linear pots with their grounded connections next to each other and electrically connected. Imagine a piece that goes between the two pots and it connected to both pot inputs via an elastomer (i.e. just wrap a rubber band around the pieces). When you move to the right, one linear pot will move freely, while the other is stuck against the hard stop. But because its connection to the input you're moving is elastic, the connection just stretches and doesn't risk breaking the pot. When you slide back in the other direction, the pot that was originally able to move will bottom out against its hard stop, while the one that was previously bottomed out will now move freely.

It's a little corny but I thought I'd throw it out there anyway. I can post a sketch if you like. :)

A ganged potentiometer is two or more variable resistors of the same or sometimes different value, ganged together on one adjusting shaft, normally used on stereo systems as a volume control so that one can vary the volume of both channels at the same time.

They are commercially available with from two to four sections.

Ganged pots are also used for tone controls, pan controls, as well as in power supplies and other electronic lab equipment.