The MS meter and Goniometer are available in hardware metering products at a price (check out the new DK Technology meters, for example), but software plogins are probably the way to go these days.
I'm a huge fan of Z-plane's PPMulator, available in the basic + version, or with true-peak and Loudness metering in the XL version.
use the BBC-style dual twin PPM display with left-rught on the red/green needles and mid-side onthe white/yellow needles. The Side needle should never be higher than the mid needle, and the bigger the difference between them the narrower the mix.
The freeware Fluxhome Stereo Tool is a brilliant audio vectorscope:
A centre mono signal would produce a vertical line, and as the stereo width is increased the image expands into a ball of string. That ball should never be wider than it is tall! Effectively the Mid-axis is vertical and the Side-axis is horizontal.
Technical Editor, Sound On Sound