How can I tell hot from neutral on the power cord of an evaporative cooler?
If you don’t have a meter trace the wires back at the fan if they do not connect to the case it won’t matter many (most) motors the wiring goes directly to the windings and only the case is grounded with the green wire.
So if no connections other than to the windings a single phase ac motor won’t matter, if it happened to be DC the fan would spin backwards if not properly connected but that should not hurt the motor, power down and switch them if the fan spins the wrong way.