Recording to test right and left channels...

Buy this CD, wonderful music, much more involving that test tracks

https://www.amazon.com/Friday-Night-San-Francisco-Mclaughlin/dp/B000002AHM/ref=sr_1_2?crid=3SZQOAC3Y...

side 2, tracks 2 and 3: all 3 guitarists play, you will hear l/r and center, and the audience noise gives clues as well.

Have now, or future Turn Table? then also/eventually get the LP version. CD: you know l/r/c info is correctly sent to system, get balance right, then knowing that, the LP, refine your Anti-Skate setting for l/r/c 

to refine the balance like my prior post, or just to verify if left is left?

first, verify the speakers are ’in-phase’ to each other. (cones moving in/out the same) (+ to + and - to - on both sides). If you cannot see the difference at both ends of the wires, then you can listen for the amount of bass and clarity of the location of sounds.

get balance, tone controls, any filters in the middle or off. reverse only one speaker’s wires. more or less bass? more bass is correct.

now mark the ends of the speaker wires + side with a red marker or red tape so you know for next time.

.......................

now: is left input going out to the left speaker?

any balance or tone control, move fully left, changes on the left? good. if not, switch the speaker wires.

no balance or tone controls: simply disconnect one of the input wires from a CD player or anything. remove left input. left speaker goes off, it’s correct. right goes off, switch the speaker wires

OP,

you did not mention LP, just how to know if left right is correct.

Done?