A spectrum analyzer is a useful tool, but no substitute for practice.
(or an EQ plugin which has its own spectrum analyzer, like
http://www.kvraudio.com/get/2747.html)
As is comparing your music to "professional" CDs (warning: there's a trend these days to run everything through a brick-wall limiter before releasing a CD, which many skilled produces call "bad" but gets done anyway.)
Its not always the right tool.
For instance, a dip around the 400hz area can produce the same effect as boosting around 6khz. Just trying to match the 6khz region of your song versus the professional CD will not necessarily make your vocals as clear as theirs, since yours might have more "muddiness" in the 400 region. So use that tool exactly as you describe, just don't trust it implicitly.
another useful tool are EQ guides like:
http://www.modcam.com/emusic/Frequency_ranges.pdf
Using a guide like that, using a spectrum analyzer, using professional CDs as a comparison are all good ways to train your ears. The bottom line is it takes practice.