Space Bar is a plug-in for the popular type design application Glyphs.app that gives you a lot more insight into the spacing of your font. It is particularly handy for working on a font with multiple masters.
All infos here: https://yanone.de/software/spacebar/
This page hosts the 30-day trial version of the plug-in to be installed via Glyphs’ Plugin Manager. The paid version will later be available via my online shop.