Question

How would you know if two vectors are perpendicular?

Answer 1

Answer:

If their dot product is equal to 0, the two vectors are perpendicular.

Answer 2

get their dot product, if the dot product is 0, then they're orthogonal (perpendicular).

also keep in mind that, if two vectors bear the same slope, namely <a,b> and <c,d>  and the simplified slope b/a  is the same value as d/c, then they're parallel vectors.