How can I tell if soy sauce is of good quality? How can I tell if soy sauce is of good quality? Is it possible to tell from the smell and colour of the sauce?

I'm aware of the differences of dark vs. light soy sauce (see this question: Soya sauce. Are they different for variety of dishes?).  There are also regional differences of soy sauces.  

Just a simple rule-of-thumb would be good.
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 .