I know I see the warning in DBM, but I also use Quartz:

http://www.wowinterface.com/download...astingBar.html

Anything that shows enemy castbars is enough to see him casting "throw saronite". I'm not sure that he actually has a target selected for the boulder or not. As soon as I see the cast start, I begin strafing, and you can see the shadow on the ground where the boulder will hit. Even if the boulder does manage to hit you, if you're team was already strafing, they almost always get thrown together, rather than all over the place.