Get it from a manufacturer's data sheet? No two valve designs have the same torque curve, that is one of the things that a manufacturer determines before putting a particular valve on the market.
There are a lot of factors that come to bear on the torque on a ball valve. The upstream pressure pushing the ball into its seat is a factor that determines the torque needed to open the valve against shut off pressure. The presence or potential for any particulates in the fluid can increase the torque requirement. These are but a couple of the factors and there are more.
So when the manufacturer has all the design parameters they can give you a good estimate of the torque required. Take that torque value and add a fat factor of safety for actuator sizing.
