By drawing a vertical line. This is called the vertical line test.
If the vertical line intersects one point in only in a graph, then it's function.
If the vertical line intersects more than one point in a graph, then it's not a function.
What does it mean?
If the vertical line intersects more that one point, it means there are two values of x (range - the first elements) that correspond to one value of y (domain-the second elements). Remember that a function does not repeat the same element/ values in domain (x).