• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Capacitors - How to install?

On older caps, the lead attached to the outside foil was considered to be the negative because it shielded the inner foil, thus being somewhat quieter. Some of the boutique caps used to (may still for all I know, but I haven't seen it in a while) mark their outside foil for the same reason. I think Percy marks his film caps with a marker to show the negative lead. How he tests them, I do not know.
Note that a film cap will function safely either way; it was just for that Nth degree of performance.
I'm not sure I follow the reasoning on bringing a film cap up on a variac. If you're confident that you've got the right cap in the right place and that the solder joints are good...turn it on.