How do you level your turntable?

A bubble level should work fine. You might want to take multiple measurements near each corner though.

Markpao- Being a carpenter- you should know that you must turn any bubble 180 degrees to confirm level. I have to send my Stabila levels to Germany every year or 2 to be dialed in. They are ALWAYS off a bit -

Pied,
I use the plinth because that's what the arm is mounted on, not the platter. Might not be the right train of thought, but it makes sense to me to level the surface that the arm is on. Example, say the platter is different from the plinth by a degree. Well, if you level the platter, now the plinth is off, and so is the arm. This could cause the arm to want to swing because it's off.

I also use just a bubble level on the plinth, and double-check it on the platter.

I have a few bubble levels
My smallest one has a magnet at the end is the mangnet bad when levelling your tt