I wouldn’t say we have no way of knowing, just that we disagree (often on edge cases). But people way smarter than me spend their lives thinking about these things and form convincing arguments is support of definitive answers.

To draw a parallel, most of human history we observed the world and reached conclusions. Mostly we were wrong but sometimes we came pretty close. Then we discovered the scientific method, which allows us to move closer to the truth over time. (Note, though, that the epistemological worry reappears, albeit in lesser form, as the scientific method must always be amendable to new empirical evidence that contradicts highly confirmed theories.) My hope is that philosophy will discover a science of normative facts, giving us an agreed upon method for determining moral and aesthetic value.