I grew up with a habitual overclaimer. He wildly exaggerated his expertise, at times claiming knowledge of things he couldn't possibly know -- people, events, ideas that simply do not exist. We're all familiar with these people who feel the need to overestimate what they know about the world. What underlies such assertions of impossible knowledge?
Ultimately, there is no way to be sure that the so-called right decision has been made before it has been made and acted on. All you can do is ensure that the process of decision-making is deliberately and painstakingly executed. That doesn't guarantee the best decision, but it sure makes it a lot more likely.