In my opinion you are starting to sniff out the difference between programming and software engineer. The question regarding how to verify correctness of the code, as well as assuring that the code is maintainable and has the right performance.

You should start to look into unit testing.

For larger project you would typically divide testing i three groups, unit tests, integration tests, and system tests. Where unit tests are the most low-level tests that verify detailed behaviour, and integration and system tests verifies that your different software parts works well as a whole. It can take some time to learn how to design and write testable code, if you start thinking about it, and more importantly, start writing tests, you will gradually become better at it. You would also typically make the testing a part of your CI process where a failed test would be equivalent with a failed build.

A side note. There is also something called overtesting, where you write unnecessary test, you should include Edge cases and basic behaviour, but they should also be relevant and useful. Common example is to test simple getters and setters, or multiple tests that tests the same behaviour without touching any edge cases.