Use your ears, you’ll get better. Mix at lower volumes (at high volumes, you’re head tends to subconsciously equalize levels) pay attention to how much you can hear different things. For me, how much the snare cuts through the mix is an indicator of how well the guitars are mixed – if the snare is not crystal clear at all times, something else is too loud. YMMV. Panning is important, too – where things are in the stereo field. Generally, bassy things belong mostly centered and trebly things panned wider left and right. The reason is that bass frequencies are omnidirectional – they go everywhere. That’s why you can put a sub anywhere, but tweeter placement is critical.
Like all things musical, it’s practice, practice, practice.