do a scratch track and then listen to it both solo'd and within the context of the mix.
leave the EQ flat. if there are things you don't like about the vocal, then sweep the EQ until you find what you don't like and cut that frequency range by a few DBs.
if your ears haven't matured to the point where you trust them, then throw the vocals into a spectrum analyzer that can show you graphically what you are hearing.
thats what i do, because i don't think i'm good enough to trust my ears yet.
for hip-hop male vocals i will low-cut anything below 120hz, for female vocals i'll cut higher, but i've only recorded one female rapper.