Orientation of "bipolar" capacitors on PCB
If you have a capacitance meter, you can wrap the capacitor in aluminum foil and measure the capacitance between each lead and the foil. The lead connected to the outer wrapper (or can) will have a slightly higher capacitance; you may be able to detect this. If your capacitance meter has a 'guard' terminal, connect this to the unused capacitor lead -- this will create a more significant change between measurements of each lead.

Note the capacitance you may measure will be quite small -- few pF, and you will need a stable and repeatable setup.