Method
Use the Vertical Line Test
Draw light vertical lines through the graph. If a line passes through more than one point on the graph at the same time, then it is not a function.

Method
Use the definition of a function
Examine the domain (xvalues) of the order pairs. If the same xvalue appears with two different yvalues, then it is not a function.

Method
Use the definition of a function
Examine the domain (xvalues). If an x is pointing to two different y’s, then it is not a function.
