Maybe a bit unorthodox but I have it plant 1-3 bugs into the code it writes (depending on the size of the MR I'm working on); they range from simple one-off issues to something more structural.

It then gives me a hint on the area or a typical code flow that would hit the bug - whichever gives me less info. I can obviously ask for more hints as I read along.

Obviously all honour-code based since I could just ask it to fix the bugs itself. I think this is something that can't be fixed systemically - either you're going to bother reading and thinking through the code or not.

Other than that I guess reading the tests - to make sure they fail without the code / fix, to make sure the setup makes sense etc.