What is the best way for a rescuer to know that a rescuer breath for an infant victim is effective?

The correct rescue breath will be just the amount of air required to raise the the chest of the victim.

It is important to make sure that the airway is open (and clear obstructions) prior to breathing in.

If the stomach rises, this indicates that the air blown in is going down the esophagus and into the stomach, which on top of being ineffective can be dangerous as the swollen stomach will block major blood vessels from returning blood to the heart resulting with heart failure.

This applies to all victims, regardless of age.