Usually you don't check your own code for security vulnerabilities. It's kind of like reviewing your own writing for grammatical errors, which can be difficult.

Usually you don't check your own code for security vulnerabilities. It's kind of like reviewing your own writing for grammatical errors, which can be difficult. It can be done but if you don't have experience to know what you are looking for, you might have a lot of trouble. Most people hire third parties to review their code. With that said, when building software you should be building security in the entire time. This comes from good programming practice, structuring your code well, using internal code reviews, and writing your code in a way that the engineers working on it reduce the number of mistakes. "Checking" your code for security vulnerabilities is not trivial and requires domain expertise in a lot of different areas which makes actually trying to answer this question kind of ridiculous.