I've read my share of frequency charts and know the general ranges that are taken up by each instrument. But for me, aside from telling when something is too thin or muddy, I feel like there are a plethora of mixing terms that I just don't get.
Anyway, what I'm wondering is if there are resources to know, when listening to a mix, which specific range to tend to? Guitars are obviously mid-heavy, but for me to distinguish exactly which section of the mids to address and how, is very difficult....does it just come with more and more practice or are their hearing exercises to know exactly what certain frequencies sound like and when they're honky or tinny or bright or brittle or [insert made-up mix term here]?
I have heard of ear training classes for engineers, but don't know of any specific ones to recommend. That aside, a good way to get to know what frequencies do what is to experiment a lot. Open up some of your past projects and start messing with the EQ controls on particular tracks. You'll get a sense for what does what fairly quickly. For example, on a guitar, try boosting the upper midrange and hear what that does to the signal, with the track still playing, lower the frequency and hear how that changes the sound. Solo the instrument if you want to hear the affect of your EQing more clearly.
If you have an EQ with a real-time analyzer function on it, that can be very helpful, because it lets you see what's going on frequency wise, and you can start connecting the dots (figuratively speaking) when you hear a particular instrument play a note or hit (for example a snare drum), and you see the corresponding change in the frequency display in real time. If you don't have an EQ with an real-time analyzer, you can insert a standalone one after the EQ, so that anything you do in the EQ will be reflected in the frequency display. Good luck!