In order to effectively validate the software, you need to first come up with detailed functional specifications on what you, as the end user, expect from the software. Once you have completed that, you basically just need to challenge each of those functional specifications to ensure that you are indeed getting what you expect from your software. Don't forget to include different users and/or different computer systems where appropriate to fully challenge the functionality.
. Anyway, i have to prove that my software always outputs 4 !)