Can pain killers actually find where the pain is or do they just act on the whole body?

Not specific: The pain killers fit into specific molecular receptors much like a lock and key. However, certain pains are not covered by narcotics. Some pain such as neuropathic pain can actually respond better to drugs commonly used for depression, fibromyalgia, and seizures.

Brain: Most pain medicines are narcotics , or opioids. They act on receptors in the brain , usually mu receptors. The brain is where pain is processed and experienced. Other types of pain killers, like anti-inflammatory drugs , work on different receptors, some even not in the brain (peripheral).