Typically, players encounter different sets of boards, but there's a lot of overlap. The gang can stagger their play, over the course of the day, comparing their experience. Before half-way, at least one gang-member is likely to have previously encountered each new board. Hence the rest of the gang can bid and play that board, double-dummy.
Detecting such cheats is a difficult challenge. Suggestions...
- Look for a pattern of successful anti-odds calls and plays. e.g. slam on 3 finesses. Dropping doubleton queen off-side.
- Gang-members performance should improve as the tournament progresses and more board-information is available to share.
- If you suspect a particular group, then examine how their performance varies, depending on who plays a board first.
- Compare the performance of suspects in tournaments without cronies.