The proving natural rights things is a little ridiculous though. Because it's a product of intelligence and philosophy. You may as well ask to prove love. By defining the argument in a particular manner, one can try to subvert the whole of the discussion without entertaining in the least the arguments of the other side. Yet much of our advancement, even in the realm of science, has been pushed heavily by philosophy and abstract ideals are an important part of the human race. Without philosophy, we wouldn't have advanced as far, it was these thoughts and ideals which really took hold and pushed further exploration. With such real world results, no one can doubt the power of philosophy or merely dismiss without effort some of the more important ideals proposed by such.
While legal "rights" vary from society to society, at the base all society is created by humans. Either humans are the same or society itself defines varying sub-species of human. If humans are all the same in the end, then there must be a common set of base rights which exist within all of us. Rejecting natural rights says that humans are a product of their individual habitation alone, that we can in fact be very different on the most basic of levels. It doesn't seem rational to me that humans are different depending on where we are from. Thoughts of that can very easily excuse things such as slavery. Instead, by examining the free nature of man it's easy to see that at heart, while legal "rights" can vary, there is a base which is inherent to us all. A man is entitled to his life, he is entitled to the sweat of his brow, he is entitled to decided for himself his actions and choices, in short; life, liberty, and property are a base by which all humans are entitled their right to. As for any concrete "proof", well prove any abstract in that context. Calling for "proof" is pointless, not because natural rights are not real but rather because of nature of what rights are and how we discover them.
Though it would be interesting for me to know if people who do not believe in natural rights believe in other abstracts or products of philosophy or even gods.