How to tell the polarity of a power adapter's wires
This adapter is just a fused transformer in a box. It gives an isolated ac voltage output. There is no polarity in such a system.  One wire will have a sinusoidally varying voltage around the other, sometimes positive, sometimes negative. Therefore you may connect the wires either way.

If you want a dc output, with one wire having a constantly positive voltage above the other, you bought the wrong item.