Typically good carbohydrates has less starch or sugars and have more fibers.
Even whole grain breads this have some level of starch which will turn in sugar. This should be taken in moderation. If you have an active lifestyle this should not be a problem. If you don't have an active lifestyle it will increase your blood sugar level. In this case the body will try to remove the excess sugar from your blood. A bi-product of this process is the creation of fat on your body. This fat may or may not go to your belly.
I believe the most common reason for belly fat is stress.
Stress creates adrenaline which is suppose to provide more energy to handle an emergency or danger. However, if no physical activity follows, the body has to remove the excess adrenaline. This removal process usually creates belly fat.
Eating good carbohydrates in moderation with an active lifestyle is the best way to keep off excess fat. It will also helps to keep stress levels low. Good sleep habits will also help to reduce stress.