How can I tell if soy sauce is of good quality?
The simplest rule of thumb is to read the ingredients: Water, soybeans, wheat, salt (and maybe something to preserve it). Other brands, may list wheat before soybeans.

"Good quality" and taste may be two different things (like wine). So, it gets a bit tricky to answer. And then there's reduced sodium and light soy which may have other ingredients.

This link may be useful. It covers Japanese soy sauce and touches on the "Grades of Soy Sauce": http://justhungry.com/handbook/just-hungry-handbooks/basics-japanese-soy-sauce-all-you-need-know .