In general, we tend to trust confident people. The more self-assured, collected, and articulate someone is, the more likely we are to believe what they have to say.
There are exceptions to this. Confidence is a skill that can be practiced and then deployed in less-than-truthful situations. This is why we are often skeptical of professions like sales and politics that practice confidence.
In general, we tend to trust confident designs. The more easy to use, accessible, and beautiful a design is, the more likely we are to feel comfortable using it.
There are exceptions to this. Good design can be applied to systems that are preposterous or unethical. However, there are few guidelines to follow when knowing what — and what not — to trust. As designers, we have a unique opportunity to make an impact on this situation. By choosing to apply our skills to meaningful pursuits, we leave a void in areas which perhaps should not benefit from thoughtful design. Furthermore, by understanding where design is being used to prop up harmful systems, we can educate and help others see through to the truth.