The direction of magnetic field lines can be obtained by Right Hand Thumb Rule
Right Hand Thumb Rule
If we hold a current carrying conductor in the grip of the right hand in such a way that thumb points in the direction of current, then curling of fingers represents the direction of magnetic field lines.
Magnetic Field on the Axis of a Current Carrying Circular Coil
Magnetic field at axis at a distance x from centre O.
where, r = radius of the coil, n = number of turns in the coil and I = current, At centre of the coil, B = μ o nl / 2 r
If we look at one face of the coil and the direction of current flowing through the coil is clockwise, then that face has south polarity and if direction of current is anti-clockwise, then that face has north polarity.