How did sheriff dozier find out who the "ring leader" was (the one trying to organize a union for sharecroppers and laborers) ?

The correct answer is:

Sheriff Dozier found out professor Tolson was the "ringleader" (the one trying to organize a union for sharecroppers and laborers) by beating up Samuel, a sharecropper.

Samuel had been present on the secret meeting aimed to organize a union for sharecroppers lead by professor Tolson disguised as a sharecropper.

"The great debaters" is a 2007 drama, directed by Denzel Washington. Based on the true story of Melvin B. Tolson, a professor at Wiley College Texas who inspired students to form the school's first debate team and went on to challenge Harvard in the national championship.