you can just tell...if you sample some bass and play it back in your sequencer, chances are that it will sound flat in the mix, so you boost it at about 80, if it is muggy then, you gotta cut it at about 250 and roll of the very bottom....
you gotta think of music in three layers, top, mid and bottom, like bass is bottom, guitar/piano is mid and high frequencys are high, lol, generally you gunna have to eq most things so if you use that as a basic formula, you can tell about on the eq range where you have to boost it, cut it...
then it is a matter of practice i guess, some ppl say if you boost something you gotta compensate and cut something out, i dont belive in that personally, but you gotta think, if you are eqing something,in order to get the sound you want in the mix, but you find you are boosting the eq a lot like 8/9/10db in order for it to cut thro the mix nicely, you are probally gunna have to cut the same frequency in your other samples, so that you can apply a more gentle eq to the afformentioned sample
that technique is used alot in dance music, house and techno especially, it is called punching, so that only one sound is coming thro at a cetain frequency, so it is clear and not muggy...
but dont over eq, that is when everything gets fucked up
i know i havent explained myself very well, but it is hard to explain, it just takes practice...
but think, top, mid or bottome, when you are listening to a sample...