you just have to learn to hear it.
try setting up a parametric with a 1/4 octave q (ime it'll be a different setting for each eq), boost by 10db, and slowly sweep around. some freqs will sound good, some bad, some will cloud things up tremendously, and once in a while you'll find something magical that simply clarifies everything the element is playing.
a good deal of the art of mixing dense arrangements involves exaggerating different spectra of the various elements in ways that allow them to poke thru without stepping on anything else, or at least minimizing the clutter.
someone on this thread began a post with "frequency masking is the problem that..." masking is *not* a problem, it is a phenomenon that can work for you or against you. when you want to increase clarity and space, you need to decrease masking with levels, eq, ambience, and panning.
but sometimes you want to use masking to your advantage, because if you have too much separation on an element, the sound becomes disembodied. if you have too much separation on many elements, the mix will lack coherence and glue.
riding that fine line is what tends to make the difference between 'sounding like a recording' and 'sounding like a record.'