Mylar cap ?
 

I was at a hamfest looking for some mylar caps for a circuit that called for
them. I got several opinions on how to tell one and finall gave up. Are
there any sure fire ways to identiy a cap as mylar and why are they bettter
than a silver mica or others. Does anyone have a short chart comparing the
characteristics? How did we get by before mylar was used?
tnx
hank wd5jfr