What is the Best Tool to Measure Cartridge Azimuth?

Good point stringbean. 

stringreen

For those who are using a mirror make sure you have one that is slivered on the same surface as you rest the stylus, or you will set it wrong.

I'm not sure why you say that. Actually, the further the silvered surface is from where the stylus sits, the more apparent any deviation from level will be. That makes it easier to set accurate azimuth, and some stylus gauges are intentionally designed just that way, such as the terrific Wallytractor. But you can set accurate azimuth either way.

Have we become so enamored with technology that we have neglected the best tool: our EARS?

First, it is easiest to set azimuth if you have a tone arm that makes it easy.  Generally the VPI tonearms do NOT make it easy unless you use the dual pivot.  IMO the dual pivot makes it crazy easy.

Then, using a small flashlight shine it on the front of your arm and cartridge and make a horizontal line on the cartridge/arm parallel to its reflection on a record.  That is your starting point; do a lot of listening.

Then using your fine azimuth adjustment, move VERY slightly in one direction.  Listen for a while, and if better continue with small movements in that direction until one of those settings gives you the most spread and separation.  If worse, do the same in the other direction.  If both worse, stay at level.

Best of all, your ears are free!

Fozy meter......

melm, I hate to say it, but for stuff like this, the ears are woefully inaccurate.  To begin with, most of us do not have equal acuity in both ears, not to mention equal frequency response sensing, and not to mention room effects, etc.  And then there's the brain...
However, I also have no beef with just sitting the cartridge squarely at 90 degrees to the LP surface.  This is a recent and radical change of thinking for me.