Salesforce offers this free SalesforceLabs AppExchange app as a start, though it appears it may simply be based on User preferences rather than actual usage.
It was released just 3 months ago (based on this Idea) with this description from SF:
You've invested in enabling Lightning and now you need to know who can use Lightning and who is switching back and forth. This app will track both! After a couple of clicks for setup, the app will automatically update stats every hour... The dashboard includes a trend report as well as several charts summarizing the totals for your company as well as a breakdown by Profile. This is your one stop shop to see who is using Lightning and who is switching back.
The screenshot from the posting (below) gives the impression that it goes past user permissions/preferences and actually queries who is using Lightning at a moment in time.
I'll be curious to hear if it actually accomplishes this.