Recording to test right and left channels...

+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.. 🤔