How to determine the voltage rating of an unknown capacitor?

[MarkT]

In a ceramic, the physics is quite simple. As the voltage increases, electrons build up on the surface of the plates, and as the voltage increases, the additional electrons form a thicker and thicker layer because they're repelled by the electrons already there. The effective plate separation therefore increases with voltage, and so the capacitance goes down. In some dielectrics it can have dropped by 80% by the cap's rated voltage.

Y5V dielectric - "just say no".

You made that up and its bogus.  High-k ceramic caps use a ferroelectric dielectric which is analogous with ferromagnetism in that the behaviour involves domains, hysteresis, saturation - ie a very non-linear response.  And electrons don't pile up at the plates into a thick layer, there's just a higher areal density of charge at the interface to match the displacement of charge in the body of the dielectric.  Charge can only displace by a fraction of the crystal lattice constant else you get breakdown.