Get the Sterophile Test CD which has a speaker channel orientation track. It will take you 10 seconds to fugure it.
I would strongly discourage attempting to play a CD on your turntable. Unless I've completely lost touch with the advancements in technology, it will not likely yield the results you need to determine which cable is which channel.
I'm not sure how proficient you are with adjustments to your turntable, but you could potentially determine which connection on the tonearm-cartridge interface is for the right channel, pull that connection (carefully) with a pair of tweezers, and play an LP to determine which is the left. Once you figure that out, it should be pretty easy to figure out which one is the right.