Another capacitor question

[Hcabnivek at aol.com]

I can tell that my polyproplene capactiors are, in fact, polyproplene caps
only because Digikey says that is what they are.  This is fine, I trust them
to be right.  My question is; How does one identify a cap of unknown origin as
to it's material type i.e. polypro, polyeth, polysty, mylar etc.  Obviously
electrolytics and ceramics are easy, and all the polystyrene caps I've seen
have the same look to them, but how do I tell what type cap I have if I don't
know it's origin?  Most of the rest of these cap types are in epoxy dipped
cases and are visually similar, correct?.  Is there a secret code that
identifies the material type.  A lot of the stuff I use to build modules is
"surplus". On occation I'll tear out a cap or pot from a piece of expensive
(and often times fairly modern) scientific equipment that our physics lab has
junked. Lots and lots of physically large, small value epoxy dipped caps in
this stuff.  How does one figure out what is inside these plastic blobs?
Thanks in advance,
Kevin

Kevin Bach
Hcabnivek at aol.com