There are a multitude of options when it comes to testing code. Input/output testing can be verified either by computer or person testing, software automation testing such as CI/CD is also a great choice for efficiency and accuracy. At the end of the project, you want to ensure you have met the client's requirements as outlined in a robust and descriptive requirements document. This ensures we have accomplished what has been asked and if we could not meet certain expectations that those are properly addressed to the client. Designing software can be a complicated and daunting task, but as long as you uinderstand the requirements and can provide a functioning program that adheres to that, provided you do sufficient testing, you should have a valid program ready for users to experience.