[quote author="toffifee"]Questions: How do I know which side connects to the outer foil (do I have to cut one cap open?)? Toff[/quote]
If you have a scope, probe, and signal generator you can put a signal on one lead of the cap, and gen and scope ground on the other. Run a healthy voltage at 1 kHz and bring the probe to the center of the cap case. Note the component at 1kHz. Now reverse the cap. The highest voltage on the scope means that config has the outer foil on the generator hot lead.
I thought there might be a convention as to the orientation of the marking on the case and the lead-outer foil connection, but the first two experiments with the same manufacturer's parts were different. Some vendors mark it with a band though, and that banded end is the outer foil.