MS processing is Mid-Side (or Mono-Stereo), as opposed to the normal LR (left-right). They are equivalent ways to send a stereo signal.
The mid channel is the same as mono, M = L + R.
The side is the uniquely stereo part, S = L - R.
Now, M + S = (L + R) + (L - R) = L + L + R - R = 2xL. So (M + S) / 2 = L.
And, M - R = (L + R) - (L - R) = (L - L) + R - -R = R + R = 2xR. So (M - S) / 2 = R.
If I remember correctly, FM radio uses MS, so that the main signal is M, and the 2nd signal is S. In the case of bad reception, at least the M comes through. Many producers use MS to check a mix for things like off-center bass or phase cancellations. Using a plugin such as Voxengo’s, you can adjust the level of the S channel while listening in stereo. S is often considered a measure of stereo width. Too much S and it sounds unfocused, too little and it sounds mono. You can also do non-EQ processing to M or to S. Hence “MS processing”.
Meanwhile, Cubase 9 has a new EQ (called Frequency) which has 3 modes - Stereo, LR and MS. In LR or MS mode, each channel has its own EQ curve. The new insert also shows the output spectrum (levels versus frequency) for each channel.
Cubase has another tool (which is less useful) called Multiscope. If you  insert multiscope on a channel,  press the “Scope” button in the lower left (it turns yellow), with  “stereo(front)” next to it, and  the amplitude cranked to maximum, then you’ll see a Lissajous pattern when you press play. The pattern shows how wide the stereo image is in real time. Notice the vertical axis is labeled M. That makes the horizontal axis S. L and R are also represented. A little cyan bar moves between -1 and +1, indicating the LR channel correlation. (+1 means L = R, -1 means the same thing, but they are totally out of phase, and 0 means they are completely independent.) You don’t want to see that bar spending much time below zero.
MS can be counter-intuitive at first. To get a feel for what MS is and is not, one of the first things I did with Cubase was project Dog-Cat. On the right channel, I recorded an impersonation of a cat singing opera. On the left channel, I recorded an impersonation of a dog reacting to the cat’s performance. Panning the channels hard right and left, then converting to MS, I found that the M and S channels sound the same. In fact, the cat’s performance is simply phase inverted. If I were to revive the project, I might look for ways to use Frequency to get the bass of both channels to the center, and the treble to the sides. Anyway, it was more instructive to listen to just the S component of well-produced recordings. S is surprisingly quiet and weighted toward the high end.
How MS Might Help You
I’m thinking you could avoid relying on visual aids if you can become familiar with MS. If you were to learn what a good S channel sounds like – what’s there and what’s not, and how loud S is relative to M – then you could use that to at least avoid serious stereo mistakes. Fortunately, you can meter the M and S channels as you would any other, which would give you some numbers on that M/S relative level question.
I wouldn’t rely entirely on MS, however. One thing I’d do in your shoes is to put the mix on monitor speakers, then wander around the room, with my good ear toward the speakers, gently shaking and tilting my head.
Suppose you have a snare drum track. The channel meter’s colored bar goes up with the drum hit, then slowly drops. The fall speed is a measure of how long it takes that column to return to zero. Go to File > Preferences >Metering. If you set the fallback speed to a higher value then the meter will be more “responsive”. With a peak hold of 500ms and a fallback of 40 dB/s, the meter on that track will come closer to showing what you hear than it would have at 3000ms peak hold and 12 dB/sec fallback.