Are film capacitors directional?

They are not polarized(as electrolytics are), but- YES; there is a preferred direction, with regards to orientation. Some helpful insights: (http://jimmyauw.com/2010/04/24/observing-inner-and-outer-foil-of-some-popular-capacitors/) (http://www.audience-av.com/capacitors/a_applications.html)

Thanks Rodman. I don't have an oscilloscope. Any other way to measure?

Bryon

Good info, Rodman.

Bryon, if you have a good digital multimeter, which can read AC voltage down into the millivolt region, you could probably use that instead of an oscilloscope. Note in the jimmyauw writeup that the amplitude scale on the scope displays is apparently 5 mv per major division (there being 8 major divisions on the entire vertical axis of the display).

Best,
-- Al