gravity - What is the proof that the universal constants ($G$, $hbar$, $ldots$) are really constant in time and space?

Cavendish measured the gravitation constant $G$, but actually he measured that constant on the Earth. What’s the proof that the value of the gravitation constant if measured on Neptune would remain the same? What’s the guarantee of its being a constant?

There are many such constants; I just took one example so that you can understand what I intended to ask. Is the value of the speed of light really a constant? Who knows that it wouldn’t change its value on other planets, or, more precisely, what’s the guarantee that the speed of light remains the same even near a black hole? If no one can ever reach a black hole, then how can a scientist claim the value of the speed of light?