A solenoid has a large number of close, insulated circular turns. The magnet at the centre of current carrying circular wire is along the axis; so when current is passed in a solenoid, the magnetic fields due to all circular turns are added and hence the field line becomes just as for a bar magnet.
Yes, we can determine the north and south poles of a current carrying solenoid with the help of a bar magnet. For this we suspend the bar magnet freely and note its ends pointing along north and south directions and mark on these ends N (north pole) and S (south pole).
Now we bring N-pole near one end of freely suspended current carrying solenoid; if there is repulsion, then that end of solenoid is N-pole and other S-pole; but if there is attraction, then that end of solenoid is S-pole and the other is N-pole.
