I would think that the best way to guesstimate it would be by using at method like dishtowel's, but I'd anchor the truck to the machine so that you don't actually roll it, just find the point where the tires are floating a bit. Back off a good 10-20* and call it good. It's going to be different for anyone depending on whats in the truck, what type of lift, tires, etc. My choptop, for example, is going to have a rollover angle that is significantly different from someone running a ranger with 4" of lift, 33's, and a fender trim.