Question

how would u know if the following would not represent a function

Answer 1

are you talking in context of a specific question or the general procedure

Answer 2

general

Answer 3

http://www.brightstorm.com/math/algebra/graphs-and-functions/relations-and-determining-whether-a-relation-is-a-function/

Answer 4

check out this link

Answer 5

If you are good at graphing functions easily, just make a vertical line anywhere, if it intersects your graph at more than one point anywhere, the graph is not of a Function.
or, check by this:
It is a function when each x of the equation has exactly one y. So mapping-{(1,2) (3, 4) (5, 6) (1, 8)} is not a function as for the value of x=1, we have two y's, i.e. 2 & 8.