posted by Kathy .
If the domain and range of a function consists of real numbers, then you can graph it by plotting all ordered pairs (x, f(x)). Remember "y" = f(x). How can you tell by looking at a graph if it represents a function?
It is easier to tell which graph not a function.
If there exists any vertical line x=k that intersects the graph at more than one point, the graph is not a function. This is call the vertical line test.