This just came up in correspondence with an author, and I thought I would share it here.
There's a quick and dirty test for knowing whether you need to hyphenate adjectives. Let's say, for example, you're pondering the phrase,
mind blowing images
and debating the hyphen between mind and blowing. In order to decide, just split apart the pieces and see if it still makes sense:
Er, not so much. You really need to link mind and blowing because these two words form one conceptual unit, so you need a hyphen. Contrast this with,
tall cold beer
which can be split into
and still make sense. In that case, no hyphen is needed.
This isn't an all-purpose rule. There are exceptions, such as
beautifully made gown
because the -ly adverb used in this compound way never requires a hyphen. And then there are borderline cases, such as
ruby red gown
in which ruby modifies red, but can also be said to modify gown. These are sometimes called squinting modifiers -- modifiers which can be read to modify two different pieces -- and are held by purists to be evidence of imprecise writing. If you want to eliminate the squint, hyphenate ruby and red. Alternately, choose either ruby or red to modify gown. But, honestly, this is one of those areas of style and grammar in which reasonable minds can differ. You might choose one method and find that your copy editor changes it on you.