How can you differentiate between function and non function mapping

To differentiate between a function and a non-function mapping, consider the relationship between inputs and outputs:

A function is a relation where each element in the domain (input) corresponds to exactly one element in the range (output). For example, if $f(x) = x^2$, then for $x=2$, the output is uniquely $f(2)=4$.

A non-function mapping is a relation where at least one element in the domain (input) corresponds to more than one element in the range (output). For instance, if a relation maps $x$ to $y$ such that $x = y^2$, then for $x=4$, the outputs could be $y=2$ or $y=-2$, meaning one input has multiple outputs.