Understanding cognitive biases, such as the anchoring effect or confirmation bias, helps designers create more persuasive and user-friendly designs. By leveraging biases, designers can guide user behavior and decision-making more effectively. To test these effects, designers can employ methods like user testing with varied design elements that play on different biases. For example, by altering information presentation to see how it influences user choices, or using eye-tracking tools to analyze how users interact with biased design elements, designers can assess the impact of these psychological principles on user experience and behavior.