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The mark on old vintage cylindrical caps indicated the lead connected to the outer layer of the foil. It is not at positive-negative polarity. In some situations it was thought the outer foil should be connected to the side of the circuit that is more closer to the signal ground (AC wise).
I have seen someone suggest you could determine the outer foil side of new capacitors by connecting one end at a time to a scope probe and hold the sides of the cap in your fingers. (Maybe you have to connect the other cap end to the probe ground lead.) The bigger signal (noise pick up) on the scope is when that end is connected to to outer foil end. I never tried that, but I see that it could work.
I don't think I ever thought it was necessary however.
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