This ability comes with experience and yet it also requires an acute sensitivity. Many experienced leaders lack that sensitivity and miss the subtle points. The best ones know that intangible factors can supercharge or severely diminish tangible ones.
Here's an exercise for your next board or committee meeting: Shortly after the meeting begins, make a mental note of what will happen and who will favor or oppose certain points. Afterwards, check your predictions. If you missed anything, identify what you should have factored into your original analysis. Keep doing this at meetings over the coming year until you become an expert at scripting the action, This may seem strange, but if you are new to this, you'll be amazed at what you'll start to notice.