If you’re no longer able to tell them apart maybe a younger friend/family member with better eye’s could help you out, Brass having a more yellow cast to it should show up more clearly perticularly if it has oxidized over a period of time.
Polish with a bit of steel wool and try a different light (brighter and/or another color temperature). That might be enough for you to tell the difference.
Do not use a Bright Boy track cleaner, it will most likely irreprerabley scratch the top of the rail causing no end of track cleaning problems…
Bright Boys do not polish they grind. It will get them clean for a short time but they willl get dirty again almost immediately causing recleaning and recleaning and recleaning etc. Instead get some metal polish (like Ma’s Mag Wheel Cleaner) and shine up the top of the rail. Brass should have a yellow gold look, while nickel-silver will be more like crome.
Except that his track is in a storage bin, not on a layout. Likely it’s very oxidized. A file will also work if you don’t mind sacrificing the last 1/4 inch of one piece.