Why never clockwise? How does it 'know' to go anticlockwise?
Answer
The direction of the magnetic field is defined in terms of its effect on a current (or moving charge). Specifically, the magnetic field points in a direction such that the force on a current will be in the $\vec{I}\times\vec{B}$ direction. (That's just a convention; there's no fundamental reason you must define the magnetic field direction that way.)
Meanwhile, parallel currents are observed to attract each other, so we know one current is producing a magnetic field and the other is experiencing it. If you work out which direction the magnetic field around the first current must point in order for $\vec{I}_2\times\vec{B}$ to point towards the first current, it's in the right-hand sense (or anticlockwise around the first current if the current is pointing towards you).
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