The sole indicator of whether or not the Boar King caught up to a Brangara puppet was his movements. It was already planned beforehand that when the second Brangara puppet gets caught, it'll speak for exactly twenty-eight seconds before committing suicide.
Therefore, if the Boar King were to stop moving for that duration, it would be an indicator of the second Brangara puppet's death.
Since the Empyrean Zingers had the Secondary Nature of Internal Inertial Gravity, they could always locate the Boar King's position through fear. And now, they were taking advantage of the exact same fear to time their actions.
The only time the Boar King stopped for twenty-eight seconds was when he faced the second Brangara puppet. Hence, immediately after that, a boulder on Fral Island shattered to reveal the third Brangara puppet.