What you have to do first is learn what freqs sound like. Buy something like the software on trainyourears.com, Auricula (android) or quiz tones (iphone/ipad). I own Auricula and have it on my smart phone. It has helped me out allot. You have to become familar with what a boost or cut at a certian freq sounds like.
A good site to check out also is understandingeq.com but this is if your a totally new to eqing. In the videos he just goes over what each knob does on an eq and then tells you how you have to train your ears to become familar with how freq sound like.
Best advice IMO, train your ears.