How do you determine the voltage a capacitor will hold?

That depend on the insulater yu put tween the plates or waterever shape metal {plate, foil}.

It will be, at most, the charge voltage, so it would not go over 50.  How high a voltage it can hold depends on the material it's made from, and when that material breaks down and bleeds current across the insulating layers.

If you do it just right, you can build a voltage doubler or tripler and truly exceed the rated voltage of the cap.  Caps are rated, but you can go over.  They are given a guarantee voltage, but this will always be under-rated.  

The voltage a capacitor can hold depends on the thickness and strength of the insulating material between the plates - the dielectric.

You can estimate the breakdown voltage this if you know the properties of the dielectric material (link below gives some examples.) This would be the "safe" way to answer your question.

You can measure the breakdown voltage using a high voltage supply (your cap will only charge as high as the supply you're charging it with.) But you could destroy your capacitor in the process. It's an interesting experiment though.

I agree with More Slack. Just try it and see You built this one, you can build another. Almost any dielectric will handle 50 volts. You will probably need a high voltage supply to actually get it to destruction. One easy experiment is put it across the line. 120 volts RMS 169 volts peak. I am assuming that you know how to take proper safety precautions.