Hello;
Since the slave board on that Labview example VI is configured to expect an external Scan clock pulse train, and later on the same example the Scan clock is defined to be one of the RTSI lines, if the slave board is acquiring any data, that means that the synchronization is in place.
To double check the synchronization visually, you can hook up a 2 channel scope, and maybe hook up two waveforms you know are in phase, one to channel 0 of the master boar and the other to channel 0 of the slave board. Then, configure the scope to trigger of channel 0, and connect channel 0 of the scope to the master board and channel 1 of the scope to the slave board.
With that setup, you will see both waveforms in phase on the scope.
Hope thie helps.
Filipe A.
Applications Engineer
National Instruments