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

Also, Bryon, if you use a battery powered DMM instead of an AC powered oscilloscope, it may be that instead of using your hand as the "signal source" you might have to drape an AC power cord (that is conducting current) over the capacitor. The 50 Hz waveform that is shown in the scope photos (he's undoubtedly in a 50 Hz rather than a 60 Hz country!) is probably contributed to by leakage between the scope's AC input and the ground of the probe, that is in turn connected to one end of the capacitor.

Best,
-- Al