Ok, select the task you are trying to identify its driving predecessor, by clicking the 'Task' tab, 'Tasks' group, 'Inspect' dropdown, 'Inspect Task', a box will appear on the left side of your screen, the driving predecessor will be noted there, its also a link so by clicking on that predecessor you will jump to that task.
I wish there was a column for this when in the 'Predecessor' tab in the Task Information indicating the driver but as far as I know there isn't, but this is a method to determine the actual driving pred.