Check whether each input has exactly one output
If any input is paired with more than one output, it is not a function
If every input is paired with only one output, it is a function
In a set of ordered pairs, look for repeated first coordinates with different second coordinates
In a table, make sure no x-value appears with more than one y-value
On a graph, use the vertical line test
If any vertical line intersects the graph more than once, it is not a function
If every vertical line intersects the graph at most once, it is a function