You could just calculate the moment of inertia of the rocket, apply the thrust, and calculate the rate to rotation that results.
Load your rocket with fuel amount and distribution as it will be at the time of firing the RCS, so that you have the mass and mass distribution correct.
Now hang the (inert!) rocket on a string located at its center of mass.
Fire the RCS.
Visually observe the rotation given to your rocket by the RCS. This will give you an incredibly accurate image of the turning impulse delivered.
Unfortunately, for a real rocket, this is only the first step.
Your flying rocket's forward thrust is irrelevant, assuming it is acting correctly, through the Center of Mass.
But, your rocket is moving. At a high rate of speed, one hopes. The air resistance is not likely to be symmetrical, especially if you have fins on the rear of your rocket!
So you take the observed turning impulse, and you calculate the aerodynamic righting force, and you play the two off against each other. This will give you the (very much reduced) amount of turn that your RCS imparts on the moving rocket.
Unfortunately, the math required to accurately model your aerodynamics is.. scary. Supercomputer-hours type of scary.
The easiest way would be to just guesstimate it, test, and adjust as needed.