Recording to test right and left channels...

XLO Test & Burn-in CD

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 

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.

I think you want   
IIsoTek   
The Ultimate Setup Disc

what are you unsure of?
Is this cartridge pin wire uncertainty?
Interconnects between turntable and preamp?
CD and preamp?
what?
If its beyond the cartridge just pull one RCA and see what sound remains (with the speaker disconnected and/or amp off of course)
Cartridge pins are a bit more complicated to diagnose.  Just make sure you follow either the color codes or the labels.  Standards can be found on the web with some simple searches.
G

I always use Take Five by Dave Brubeck you should hear the opening bit with cymbals coming out of the left channel.

Always easy to identify.

Here’s another burn in CD and they have a LP version, too.

https://www.amazon.com/Cardas-Acoustics-Irrational-Efficacious-Burn/dp/B00OY8UXME

+1 on the XLO disc.

If you're streaming search for something like "stereo test speaker setup"
One on tidal checks phase, frequency response, etc etc

Yes, like the tip "Take Five by Dave Brubeck".

Or if you prefer Adele track "send my love".
There she is saying "only the guitar thank you" in the left channel just before the track starts. (Another thing is that she is saying it in a rather low level so you can check that you have the resulution in your reproduction system or that the room is quiet enough, to be able to precive each word that she is saying.)
Good luck!

OP,

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

Done?

elliottbnewcombjr the example tracks are available on all formats:
 "Take Five by Dave Brubeck".
 Adele track "send my love"

And then there is LP calibrations/reference test discs. But I find Adele to be preferable.
But LP test discs is better in other areas than just know what is left or right channel.. 🤔

PS Audio has a great CD/ files with an accompanying book. Check out their Octave Records section. In includes phase checks and much more. Comes as SACD/CD with or without book. 
Based on your question, this would be a great way to educate yourself and tune up your system. 

Second that.
The disc from ps audio has something that is little bit unusual. There is tracks were the performer is recorded and documented with different distances with further and further away from the microphone.

That is great because 99.5% of the time we have no information of what distance a artist/instrument is from the microphone. And can be good to train and to be able to hear the differences.

Yeah you can hear if it is mounted in a acustic guitar body (no room information) or if in some of the cases there is information that a trumpet were further away than the normal during recording. So you get the feeling that it is in the far so to say.