Question

How do you know whether or not something is a function?

Answer 1

depends on what information you have
ordered pairs?
equations?

Answer 2

For a function, every input has only one output, but different inputs can have the same output.

For example:

Is a function because every input has one output.
However:

is not a function because the input 2 has multiple outputs.

Answer 3

a function must also pass vertical line test