It helps a lot if you use a strongly typed language with a strict compiler and get the LLM to write plenty of tests. Then you need to understand that the tests are logically correct. The LLM is also good at documenting the functions, so you can review that matches your intentions and the code as well. Also the LLM will pick functions from your own source code library to compose new programs for you. So the reuse of your own well tested code should increase confidence.