Method 1 Using Magnetic Compass

1. Get a magnetic compass and bring it near to one of the poles of a magnet without touching it.
2. Observe carefully where the needle of a magnet is pointing. The point that normally points to the north pole of Earth should point to one end of the magnet.
3. That end is actually the South pole of the magnet.

Method 2 Using Thread and Stand

1. Bring very thin thread and stand.
2. Tie a bar magnet at the middle of the magnet and suspend it on the stand.
3. Wait until the magnet comes to the rest position. 
4. The end pointing the geographic north of a magnet is the south pole of a magnet and vice versa