I believe you'd answer this question with your own explanation, such as
"When shown using a table, a sign that show that the representation is not a function includes when two x-values are repeated with different y-values."
"When shown using a mapping diagram, a sign that the representation is not a function includes when there are two arrows coming out of one x-value."
"When shown using ordered pairs, a sign that the representation is not a function includes when there is repetition of the same x-value with another y-value."
"When shown using a graph, a sign that the representation is not a function includes when the graph does not pass the vertical line test."