If you don't have one already, get yourself an inexpensive digital multimeter. It will be indispensable in future electric flight endeavours. You do NOT want to get the polarity reversed, and the only way to be 100% absolutely sure is to use a meter to check.

Simply pick a way, and connect one probe to each wire. If the meter reads negative, you've got it backwards, so switch wires. When the meter reads a positive voltage, the wire touching the red probe is + and the one touching the black probe is -.