Simply put, what you build should be working. You should Write tests. And you should make sure your end goal is working. So manually test your changes, and make sure they are working.
If you are lucky to have a Quality Engineer in your team, Reduce the feedback cycle by testing your contribution thoroughly before sending for review or for someone else to validate what you did is working. If you don’t, the end user might be the one finding it out, and that’s never fun.
Part of Tips for Software Engineers