Answer:
Vertical line test.
The vertical line test is a visual way to prove that a specific curve represents a function. Moreover, a function is define as a relation between two sets which must associate an element from the fist set, to an element of the second one. An important fact is that the elements of the first set must have a unique element of the second set, and this characteristic is better shown with a visual method, the vertical line test.
So, if the vertical touches to points on the curve, this means that the first set is related with two elements of the second set, meaning that the curve is not a function.
Therefore, in this case, if the graph of an inverse only touches one time the vertical line, means that it's actually a functions.