It's not your job to find bugs. If you do that's great, but the author should have done that. It's your job to ensure the code is well structured, understandable, and follows team norms.

Ask questions if you're unsure of why something was done the way it was. Sometimes even just asking "why'd you do this" will spark a great design conversation. And if something is confusing it's worth exploring more clear ways to implement it.