With trunnion mounted ball valves, you'd need some info from the manufacturer. But with floating ball valves it's simply a function of the pressure differential on either side of the ball. Apply the delta-P to the effective cross-sectional area of the ball's upstream side to figure the force, then apply the resultant force to the sealing area on the downstream side. Spring-loaded seals will complicate this analysis somewhat, but the manufacturer should be able to provide some data on that. A well defined problem is half solved.
