Recording to test right and left channels...

You should have a test CD as stated above, but if you don't, on Brubeck's Time Out, the drums are recorded to the left.  Same on the CD and the LP.  

Here ya go. Start from the beginning. 

https://youtu.be/zhneUPOv1JE

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

The first Stereophile test disk has left/right tracks, in-phase and out-of-phase tracks, and test tones descending into the deep bass.

Vinyl test record or stereophile test cds or many other audio grade test discs and records too many to list.