How do we determine the polarity of dna strand?

Double-stranded DNA is a right-handed helix formed by two individual DNA strands aligned in an antiparallel fashion (a right-handed helix, when viewed on end, twists clockwise going away from the viewer). Antiparallel DNA has the two strands organized in the opposite polarity, with one strand oriented in the 5′-3′ direction and the other oriented in the 3′-5′ direction.

The 3' and 5' refer to the carbons on the deoxyribose Hope this helps :)