Final answer:
You can tell when a rescue breathing for an infant victim is effective by observing a visible rise of the chest.
Step-by-step explanation:
The rise of the chest is the most visible sign that rescue breathing is effective for an infant victim. When performing rescue breathing on an infant, you should watch for a visible rise of the chest with each breath, indicating that the air is reaching the lungs and oxygen is being delivered. This is an important indicator that the rescue breaths are effective and helping to support the infant's breathing.