In Combining Typefaces, Tim Brown outlines a process for choosing complementary typefaces. Part of this process includes comparing text set in different faces, both at macro level (to gauge characteristics such as texture and contrast), and at micro level (to examine the compatibility of the finer details).
To make comparisons easier at micro level, I created a basic online tool, Compair. Simply choose 2 fonts, some sample text, and compare features side-by-side such as x-height, cap-height, and character width.
At the moment, it’s pretty basic, but I’m hoping to add more kinds of micro comparisons (e.g. overlay the two faces), add macro comparisons, and perhaps integrate with the Google Fonts API.
Related: Upping Your Type Game . Jessica Hische