What is a good way to test if the bar is at the spine of the scapula while Squatting?

Keep your palms as close as your possibly can.

Before you take the bar out of the Rack, Position it on your back, like a High Bar Squat.

Then, Roll Your Back up slowly, while fully in contact with the Bar. You can FEEL the bar get into a SLOT. The bar is then held between your Traps and your Deltoids…this is the Correct Position.

Also, as you are performing the Squat (I assume of course, that you are doing Low Bar Back Squats), if the bar moves up during the set, you have set it up higher than it should be. If the bar rolls down, during the set, you then, had it set up lower than it should be.

If it stays in place, you have it setup Correctly.