Face detection has to do with focusing and is performed by the camera, not the flash. Whatever face detection features your camera body are capable of will determine the performance of the camera/flash system in this regard. With most bodies this would require using Live View to use the facial detection features. The EOS 1D X has an advanced metering system with RGB exposure meters rather than the usual monochrome metering sensor. This metering system can distinguish different colors/tones, detect typical flesh tones, and adjust exposure accordingly based on metering via the viewfinder as well as by using Live View Face Detection. The EOS 1D X uses this system to recognize skin tones when using PD AF so it is quite possible that when coupled with the 1D X the 600EX RT can adjust exposure to match skin tones without shooting in Live View (see the fourth bullet point here: http://learn.usa.canon.com/resources/products/eos_1d_x/eos_1d_x.shtml).

Once the camera has selected the point of focus it does communicate distance information to the flash for use in figuring the needed amount of flash power. Although the primary factor in figuring the power is the amount of reflected light from the pre-flash recorded by the area of the camera's metering sensor associated with the selected point of focus (which may have been selected by the camera using the facial recognition feature), the distance information is also used to allow compensation for highly reflective surfaces that would otherwise fool the metering system during the pre-flash measurement.