Is there a technique/tool to assist with rear derailleur high, low, and index alignment? Everything I read gives this picture like this:



Yet the reality of this alignment, it can be somewhat difficult.  Is there a better technique or tool to help get the high, low, and index shifting alignment perfect?  Or is visual vertical alignment the only way?
Well, the first thing to check is if the derailleur (usually the hanger) is bent. There is a tool for the case of a bent hanger, namely the Park Tool DAG-2. You should also visually inspect for the derailleur pulleys to be aligned in the right way. 

Then, you do the standard derailleur adjustments, which you'll get right after practicing (limit screws, B-screw, indexing). 

The whole thing becomes a combination of visual inspection and testing and practice, as DRH remarked in the comments. But most cyclists who are going to do their own maintenance will have to deal with this a bunch of times in their life time anyway, so they'll get used to it. And cyclists who don't do their own maintenance will likely just be fiddling with the indexing adjustment a bit and if something more is necessary (e.g. if they had a crash), they'd head to a shop.