How to know it is clockwise or counterclockwise? It depends on the observer's direction of viewing it. If I see it clockwise, the other person standing opposite me will see it counterclockwise. Who is true?
The quantum property called "spin" is not exactly the same as what we call spin at the macro-scale. For one, it can only have discrete value (1/2 or -1/2), meaning while it can switch direction, it can't change magnitude. But to answer your question, the spin direction is measured relative to the election's magnetic dipole. So going back to the Earth example, it's related to what direction the Earth is seen rotating relative to its magnetic field. If we look down at the Earth from above the North magnetic pole, it rotates counter-clockwise, but when looking from above the South magnetic pole, it appears to rotate clockwise. But since we can tell which direction the magnetic poles are aligned, we would calculate the same direction of for the spin of the Earth.
