Canon Speedlite 600EX II-RT Manual (en)

In radio transmission wireless shooting, you can fire the test flash and 
modeling flash (p.47) from a 600EX II-RT set as a slave unit.
Display <N>.
Press the slave unit’s function button 
4 to display <N>.
Fire the flash.
[Test flash firing]
Press the slave unit’s function button 
3 <Z>.
[Modeling flash]
Press the slave unit’s function button 
2 <>>.
A flash signal is sent from the slave 
unit to the master unit, and a wireless 
system test flash or modeling flash is 
Test Flash and Modeling Flash from a Slave Unit
Modeling flash is not possible from a slave unit with cameras released up 
to 2011 or with EOS REBEL T6/1300D or EOS REBEL T5/1200D.
For the precautions related to modeling flash, see page 47.
When C.Fn-02-1 is set on the master unit (p.113), modeling flash will not 
be fired even if you select <M>.
When there are two or more master units (p.72), the flash signal is sent to 
the main master that has the <D> lamp lit in green.