How does the doctor know if the diagnosis is c-section complications?

For what?: Your question needs more information to be answered, the diagnosis for what is c/s complications. For your pain? For bleeding problems during c/s, after c/s? Some things are quite obvious- like "due to complications we got into your bladder during the c/s" this is a risk of surgery but very uncommon- if there is a better explanation like extensive scar tissue that led that way its more likely.

Experience: A qualified physician not only knows how to avoid complications, they must also know how to recognize them & fix the problem. Whether it's from infection, injury, hemorrhage, or trauma; an experienced ob/gyn knows how to keep you safe, correctly explain the complication, and help you understand what it means in regards to your health. A good doc will tell you exactly what happened.