Levelling turntable

A torpedo level would be good.
They're short enough to sit on the platter, and big enough to read easily.
Stick one on the platter and set up in one direction, then rotate the platter 90 degrees and adjust.
Double check and make any minor adjustments and you're sorted.

The image below is a Stanley Fatmax Scaffold, and they're very good

 

This method gets my vote also. Tells me exactly if/where I have to adjust. “Keep it simple”. “Less is more”.

 

Yes, this. A bubble level gives you a reading at one point. A small torpedo level will look at the plane of the entire platter. As noted above, check in one direction and then at 90 degrees to that direction. And put the level on the platter. That's what's important. Of course, the plinth and the platter should be in the same plane, but the record sits on the platter.

 

Well, I bought a Stanley 200mm torpedo level, it passed the 180deg test in the store, got it home and 5 minutes later have a perfectly level turntable. Many thanks.