Reports on users not using Lightning

I just started at a new org and we're moving the team to Lightning.  At my previous company I had built a report to identify which users were not switching over to LE and for the life of me cannot figure out how to recreate it.  I'm tossing together a dashboard for specifics related to who is/isn't using LE, load time for pages, and what pages people are switching from LE to Classic on.  Any help with how to identify which specific users are/are not using LE would be amazing #Lightning