What Do You Hear When You Adjust VTA?

I think of it in terms of the rear of the arm because that is what is happening to change the VTA. The stylus/cantilever acts as a fulcrum or pivot point in this adjustment.

1) raising rear of arm ~ more crispness of highs and sharpened bass

2) lowering rear of arm ~ less air in the highs and more boominess in the bass

Focus on listening to the vocals, the goal is to find the setting that results in no excessive sibilance nor boom in the body of the voice; get the vocal clear and natural with the best annunciation quality and the rest will follow.

Also do this with 180 gram disc; this is a good average weight that should work for most other disc thicknesses.

Read Lloyd Walker's article on fine tuning your turntable. Be careful not to listen to a single instrument or just voice. Instead, listen to complex music (large ensemble jazz or orchestral). You also need to listen for soundstaging and resolution of inner detail. What Stevecham describes is good, but you can get mislead listening only to voice.

I actually listen for the surface noise on the record moving to a different spatial plane from the recording.

I tuned it to the sound of a Cembalo in the midst of an orchestral Bach suite with excellent success. On an Eterna digital recording no less. The VTA was right, when the cembalo sound became clear and airy and the soundstage opened up both in depth and width.
What Viridian has mentioned is exactly the side-effect of a properly set VTA in my experience.