Question

how do you deduce that two vectors are perpendicular

Answer 1

suppose two vector are A and B.

then,

A • B = 0

implies

A is perpendicular to B

Answer 2

in scalar product or dot product the two vector are equals to 0 than they are perpendicular to each other
in vector product or cross product the two vectors are equal to 1 than they are perpendicular to each other.......................................hopes it's helpful