Question

how to know polarity of a solenoid

Answer 1

To do this, point your right thumb in the direction that the current travels in the coiled wire in the solenoid and curl your fingers. The direction your fingers point will determine the direction of the magnetic field created by the current. Alternately the right-hand-grip rule is easier to use with solenoids.

Answer 2

Maxwell's cork screw law could be used where if the direction of the wounded coil is in clockwise direction then that act as a South pole and if the wire wounded in anti-clockwise direction that part act as a North pole.

Hope its useful.