Quickest way to find "neutral" colour?

To illustrate why it is important to find the pixels that need to be neutral, and not the pixels that are neutral right now, try color correcting this image by finding the neutrals and then clicking on them with the grey eyedropper (don't cheat, click on the pixels that are neutral right now):

I wonder if a Saturation Map created via a Selective Color Layer (setting all six colors to -100% Black, Whites, Neutrals and Blacks to +100% Black) may not help visualize »neutrality«. 

The problem is that you should not click on a spot where the RGB values are the same (nothing would change if you did), but on a spot where the RGB values should be the same (after you have corrected the colors).

For most images, the average color of all pixels should be grey. That is how white balance in a digital camera works, for example. Here's a method that uses this principle and often (but not always) works well:

Step 1: Duplicate your image to a new layer

Step 2: Run Filter - Blur - Average on the new layer

Step 3: Add a Curves adjustment layer above this and click with the grey eyedropper anywhere in the layer below

Step 4: Delete the blurred layer (or just turn it off)

 Often, the area in the image that is closest to neutral gray is difficult to locate. In that case:

1. Add a new Layer to the image. Layer>New>Layer
2. Fill it with 50% gray. Edit>Fill: Contents 50% Gray. Blending Mode: Normal
3. In the Layers panel, change the Blending Mode to: Difference.

The darkest point in the resulting image is the Gray target.

The next step is to locate and mark that point.

1. Create an Adjustment Layer>Threshold
2. The Adjustment Layer may automatically default to a Clipping Layer, as evidenced by the down-arrow appearing in the layer. Unclip it.
3. Shift the slider to the far left then inch it right to find the first tone.
4. Choose the Color Sampler tool. Click on the tone that appeared.
5. Cancel the Adjustment Layer and delete the 50% gray layer. The Color Sampler marker will remain.

If you look at the Info panel you will see it has added a segment marked #1 showing the original RGB values of the chosen spot followed by a slash and the new, adjusted values. For a perfect neutral gray, those three adjusted numbers should be alike. (If the #1 panel shows CMYK or Lab values, click on the eyedropper in that panel and choose RGB.)

Use Curves to change those values so they all match, indicating neutral gray.

  NOTE: At times, although precise gray makes color rendering more accurate, it is not the most aesthetically pleasing interpretation of the image. It would be prudent not to let numbers overrule taste.

I haven't used CS6 for a while, so it's hard to remember what worked back then. I did find this thread that says the sliders don't work in CS6 with the Highlights (which I would assume would be the same issue with Midtones). Maybe instead, set Select to Sampled Colors and use the eyedropper to sample an area that you would consider a neutral midtone. The pixels similar to the sample will show up in the preview. 

Have you tried using Color Range (Select > Color Range...)? You could set the Select dropdown to "Midtones" and adjust the Range to be very narrow.

Photoshop can't know what is supposed to be neutral (but isn't). Only you can do that.