Nikon D6 Reference Manual page 711

Flash Info for On-Camera Units
When a unit that supports unified flash control (the SB-5000,
SB-500, SB-400, or SB-300) is mounted on the camera accessory
shoe, flash info can be viewed in the camera display. To view
flash info during viewfinder photography, press the R button
to activate the information display ( 0 1026) and then press the
R button again.
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Flash Info for On-Camera Units