How Can I Test High Voltage Doorknob Capacitors?

I would like to test 30 or more of these capacitors (HEC HT57 100pf 15KVDC N750) to see if they are as good as new.

They are used in a tank circuit of a Colpitts oscillator, 4 in parallel, and I don't trust substituting them one at a time for a good evaluation. The failure modes that I suspect, I guess, are 1) they have gotten hot or for other reasons have begun to loose integrity in the silver-soldered connection between the plates (accross a ceramic barrier), or 2) possibly some leakage may occur indicating a coming breakdown, and explosion.

The oscillator operates at around 22Mhz, and the power consumption of the load is around 1.8KW. I suspect that a simple static test for leakage at high voltage and the use of a capacitance checker would not be enough to be sure they are fit for the work they will need to do. (The load is an induction coil for an induction furnace using a 5868 at the 500ma maximum capacity, 40% duty cycle.)

I would greatly appreciate some simple direction in how to make this test, and some idea how to evaluate how close I am testing them to their specification limits. perhaps a HAM Radio operator has some experience with testing these?

I have tried googling for test methods of this kind of capacitor, but found nothing usefull.

I don't want to spend a lot of money on testing these, of course, but maybe diverting some of the input of the tank circuit to a capacitor in test, maybe in series with a resistor or somethng and doing some measurements would work. Anyone have experience with doing this kind of thing?

Thanks for your help!