A: By navel-gazing.
I jest, but not completely: It'll probably have to be by philosophical introspection. You'll have to look at society -- societies, all over the world and throughout history -- and decide for yourself which of their traits are expressions and which violations of human rights. Figuring out from that which of those rights are "natural" and which not is... Not easy, so I'll leave it as an exercise for the reader.
But then how do we decide which rights those are? GGP suggested we do so by analyzing society, but that fails when rights are being violated, so...