The people over at Sonic Bids have put together an awesome EQ cheat sheet that will help you narrow in on what frequencies you are looking for while mixing. Of course it covers guitar, but it also covers bass, drums, keys, and even vocals. You will still need to use your ears, but this is a good outline for where to start. Go check it out!
http://blog.sonicbids.com/the-ultimate-eq-cheat-sheet-for-every-common-instrument
