Why compare with a reference?

Your ears get used to your own mix. A track you know shows at once whether you have too much low end or too little air.

Press Command-D: one in mid, one in side, for example.

Spectrum analyzer

See which frequencies are in your mix right now, from 10 Hz to 22 kHz, low on the left and high on the right.

Compare with a reference track

Drag a track you like onto the spectrum, or choose a file. Its balance is drawn as a white line, as loud as your mix. Where your mix lies above the line it has more of that frequency, where it lies below it has less.

Read it under the pointer

Point at the spectrum and it reads out the level, the frequency and the note with cents at that spot. With Peak Hold it keeps the highest level of every frequency while you point at it, and names the main peaks.

Settings

Channel: stereo, left, right, mid (the mix in mono) or side (what is wide). Or two at once: left and right, or mid and side.
Speed: calm (about a second and a half), following the music, or every hit.
On Hover: keep running, freeze, or hold the peaks.

Questions

Why compare with a reference? Your ears get used to your own mix. A track you know shows at once whether you have too much low end or too little air.
Two spectrums? Press Command-D: one in mid, one in side, for example.