Tom's first lead is the Double Four , Four/Trey , Four/Blank , Four/Ace follow.

Tom's second lead is the "Cow" Four/Six , John plays the Deuce/blank tell his partner that "Deuces" can only be lead twice. Don plays the Five/Blank as he has no help. if Don plays any other domino, it is a "False indicator". Game is not used as an indicator.
If Don could indicate:
The Five/Deuce Five/Ace would Indicate he had the Double Five .
The Six/Blank would Indicate he had the Double.
The Four/Blank would Indicate he had the Double Four
They would be a False Indicator if played in this hand.