Packaging polarity indication of a supercapacitor (polarity indicated by arrows)
Since the product you purchased has no background information, you can't be certain. However, the convention for these stacked-disk type capacitors is polarity mark points to negative lead. This is the same as is the convention with conventional electrolytic capacitors.

For example, the Eaton KR-5R5V474-R:



Has its datasheet show:



Similarly, for the Panasonic EEC-S0HD224H:



Has the same arrow convention considering the asymmetric leads with polarity indicated in its datasheet: