I think this will be a political Officer Ability. The model may attempt to reveal a sleeper agent in a unit. Individuals cannot be sleeper agents and no unit can ever have more than 1 sleeper agent. The Political Officer must be within 8" of the target model and, after spending a combat action, must win in a comparative Willpower test with the target model (both players roll 2d6 and add the Willpower score of their model). If the Political Officer wins, then the target model switches sides and activates as an individual immediately and for the remainder of the game. If the Political Officer fails, then he cannot attempt to find a Sleeper Agent in that unit again (other units can still be targeted).