Testing Metal Film Capacitors

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Original poster: "Chris Fanjoy" <zappyman-at-hotmail-dot-com> 

   I want to put together an MMC, to use as the tank cap on the TC I'm 
building. Because of the tight budget I'm on, I really can't afford to 
spend money on a good pulse cap or anything like that. Instead, I want to 
build an MMC with the old caps I have laying around.
   I have literally hundreds, if not thousands, of metal-film capacitors 
pulled from various electronics. Most are probably about 15 - 25 years old, 
and nearly all of them are the "tear-drop" shape. There are a wide range of 
voltages and values, such as 0.068 / 1600V, up to 4.7 uF / 250V. They come 
in many different colors, by many different manufacturers. Most are 
generic, some bear the Matsushita logo and there are even a few Sprague 
"orange drop" types. The big questions is - how do I distinguish the 
polypropylene types from the ones with other dielectric materials? 
(polyester, mylar, etc). Is there any kind of test critera one can use to 
determine if a cap is suitable for a TC? Perhaps some kind of test jig, 
that would apply an AC voltage of 3 - 4 times the capacitors rated value? 
(and if the cap doesn't short/explode within a certain amount of time, 
consider it a pass?)
   There must be some way to make use of all these nice old caps. Or maybe 
I'll just have to build an MMC anyway and see how it fares.
   Thanks for any advice.