In addition to the LCD overlay in the viewfinder, covered well in this answer, some Canon lenses are focus-by-wire. Any of the STM series of lenses will not move focus elements when there is no power being supplied to the lens by the camera. From Bryan Carnathan at The-Digital-Picture:
Additional typical Canon STM AF benefits include internal focusing, non-rotating filter threads (very beneficial for circular polarizer filter use) and FTM (Full Time Manual) focusing. FTM focusing has one caveat – that the shutter release be pressed half-way for FTM to be enabled.
The STM system is an electronic focusing design that needs power from the camera to function, even in manual focus mode. The camera must be powered on and the meter must be live for manual focusing to be enabled.
There are also a few other focus-by-wire lenses in the Canon lineup. The legendary but now discontinued EF 200mmm f/1.8 L was one. The EF 85mm f/1.2 L II, EF 1200mm f/5.6 L, and EF 200mm f/2 L IS are some of Canon's current non-STM lenses with focus-by-wire systems.