In formal logic, it is known as "modus tollens"... if 'if x then y' is true, then 'if not y then not x' is also true. The inverse is not necessarily true, however: 'if x then y' does not mean 'if y then x'.

In the case of a an X that is hard to figure out on its own, and it is easy to figure out Y, then it might be worth testing Y first, even if you will only get useful information if Y is false.