Maxwell's right-hand grip rule states that “if a solenoid is gripped by the right hand in such a way that the fingers are in the direction of the current flowing in the wire, the thumb will point to its north pole.”

The opposite pole will be the south pole. 

The direction of lines of force outside the magnet is from the north pole to south pole of a magnet.

Hence, Maxwell's right-hand grip rule is used to find the direction of magnetic field lines of force around a solenoid.