One can determine if a relation is a function by graphing the relation, drawing a vertical line on the graph and then checking whether the line crosses the graph at more than one point. If the line crosses the graph only once, the relation is a function.
If the relation is represented in the form of a T-chart listing x- and y-coordinates, the two columns are examined closely to ensure no x-value is matched to more than one y-value. If there is a one-to-one correspondence from x-values to y-values, the relation is a function.
To decide whether a relation is a function, when expressed as an equation, one solves for y and notes whether the equation has more than one solution. If there is only one possible correct answer, the relation is a function.