As of now, one of the most popular spacing visualisations for multiple-master type design is now open source: Space Bar by Yanone. Read all about this extension:
Here is what Yanone says about it himself:
Space Bar is a plug-in for the popular type design application Glyphs.app that gives you a lot more insight into the spacing and other metrics of your font. It is particularly handy for working on a font with multiple masters.
The tool visualizes metrics values that cannot otherwise be seen while editing. Because you’re working on a master, you can’t see how your changes play out across all your defined instances.
As with many of my other tools, present and past, Space Bar only makes information visible and does not suggest a particular fix.
As so often, I started developing it as a tool for my own design work and then found it useful enough to make it into a product. Many years ago I had a similar tool named Autopsy that would render the metrics values of selected glyphs into a PDF for review. But modern type editors allow for real-time illustration of values, so I thought it could be of much help to look at them while designing and spacing.
To this, we have nothing to add, except: go and try it yourself. :-)
Install Space Bar
The plug-in and its source code are a free download now. All you need to do is to go to Window > Plugin Manager and look for Space Bar (scroll down or use the search field), click its Install button, and when the download is ready, restart the app. If you had a previous installation of Space Bar, make sure to remove any old versions of the plug-in that may still be lurking around.
Activate Space Bar through View > Show Space Bar, access its options in the context menu (right- or Ctrl-click).
German designer, coder and artist Jan Gerner, better known as Yanone, has created several font families, made a movie, and written lots of code, among which two popular plug-ins that are available for Glyphs: Speed Punk and Space Bar, both of which are now open-sourced and freely available via Window > Plugin Manager.
Many thanks to Yanone for making this possible, and Google Fonts for supporting the project. Special thanks to Dave Crossland for making this happen. The type design world will be a better place now.