Specialists use a phantom model of the human head and body to measure SAR values.
What is a SAR value?
Specific Absorption Rate (SAR) is defined as the amount of absorbed non-ionizing radiation power (or rate of absorbed energy) by unit mass of biological tissue.
It is worth noting that manufacturers measure the SAR value of their products under laboratory conditions which often don’t reflect how we use them in real life.
This is why manufacturers advise that you use your phone at a distance from your head or body. Every phone has a specified SAR value. The lower the SAR value, the better.