Show bids require private agreements/understandings between partners. They are unfair and are not tolerated in sanctioned tournaments. The new N42PA rule forbidding their uses will be difficult to enforce, much like making other new rules forbidding private indicating agreements/understandings between partners. Such rules rely largely on player honesty and integrity.
Some players use only the two traditional methods to indicate doubles: sluffing the double itself to indicate the next highest domino in that suit is held, or sluffing a domino whose high end indicates the double held.
The first method is intuitive (players don't normally sluff a double unless they're also holding the next highest domino in that suit). The second method is risky because false indications can occur with undesirable consequences. Both methods, when used, are observable by everyone at the table, and their meanings understood by most players.
Other less common methods like reverse indicating (high end of sluffed domino indicates double NOT held) or double reverse indicating (both ends of a sluffed domino indicate doubles NOT held) are also used by some partners. Yet another method is to use the low end of the sluffed domino to indicate the double held in that suit. These methods are not universally recognized, and private conversations between partners are often necessary to agree on which method will be used in competition. In some cases, one particular method is understood to be the default when players have been partners for a long time, or their local playing groups indicate that way.
And then, of course, some players don't indicate at all, adding a sixth variable to the mix.
Without some private agreement or mutual understanding between partners, it is not likely that a player can effectively indicate to his partner which indicating method will be used. And, with a private agreement or understanding between partners, there is no way for their opponents to know which method is used when first observed. This constitutes an advantage for the team using that method, and a disadvantage for their opponents who are not privy to the private agreement(s) or mutual understanding(s).
A recent poll showed 76% of 41 participants voted that privately agreed indicating methods were not fair play; however, there was no consensus to remedy the situation. Forbidding players to have private agreements, requiring partners to divulge their private indicating method(s), and permitting players to ask their opponents which method(s) they will use were considered as possible remedies. (To do nothing would not mitigate the issue.)
Here are my thoughts for a remedy: Players may ask their opponents, before start of competition, at the table, which indicating method(s) they will be using in the game. Failure to ask prior to both teams drawing their dominos negates the right to ask. (See also suggested preamble.)
Publishing a list of legal indicating methods would be helpful, too. See follow-on document.