How do you spot sandbagging, and what do you do about it?

Sandbagging is claiming a lower standard than you play at, to win a lower band. Signs: a placement well below the claimed external rating, a run of one-sided wins, and a player who declines to move up when asked. The fixes are structural. Read the conservative end of a claimed rating, give a new player a first challenge that reaches their whole band, and let the organizer move a player between bands with the move logged in public. How placement reads a claim.