Millenium makes a gridded, transparent acrylic block for this purpose. It's admittedly expensive at $85 but remarkably convenient for establishing what's parallel and perpendicular to the record surface, with more precision than mere eyeballing. I use mine all the time; it's one of my favorite audio tools.
Here's a link: VTA block
I set the headshell parallel to the record using the block, establishing a frame of reference, then adjust by ear. My tonearms all have calibrated VTA adjustment, so standardizing the preferred setting above or below horizontal for each cartridge is pretty easy. If things get out of whack, I can quickly go back to horizontal and readjust by the predetermined amount. Good for azimuth, too.
Stacked business cards, etc., also work but they're seldom perfectly flat.
Here's a link: VTA block
I set the headshell parallel to the record using the block, establishing a frame of reference, then adjust by ear. My tonearms all have calibrated VTA adjustment, so standardizing the preferred setting above or below horizontal for each cartridge is pretty easy. If things get out of whack, I can quickly go back to horizontal and readjust by the predetermined amount. Good for azimuth, too.
Stacked business cards, etc., also work but they're seldom perfectly flat.