Unfortunately, potential supply cannot be observed directly, and inferring its behavior from variables that can be observed is a daunting task. But the task is essential nonetheless: Changes in potential supply have been among the most important influences on the behavior of the economy over the past ten years.
I should note that the course of aggregate demand is not independent of the course of potential aggregate supply. Both economic theory and empirical evidence suggest that households and businesses make decisions about spending with an eye to future incomes and sales, so that a rosier long-term outlook tends to raise demand today.