Question

how can i tell if a graph is a function or not

Answer 1

vertical line test

Answer 2

draw a vertical line. Move this line anywhere on the graph. If it intersect more than one point, it is not.

Answer 3

I guess you mean "How can I tell if it's a \(\textbf{function of x}\). In that case, just make sure that any vertical line you draw through the graph passes through the graph only once.