Final answer:
Doctors use x-rays to assess growth plates in the bones to determine if an individual has reached full height; closed growth plates indicate no further bone lengthening.
Step-by-step explanation:
The doctor can determine if Sandra has reached full height by using x-rays to look at her skeletal system. Specifically, the doctor would examine the growth plates, also known as epiphyseal plates, in Sandra's long bones. When a person has reached full skeletal maturity, these plates will have fused and ossified, indicating that no further growth in bone length is possible. This typically happens by the end of adolescence. The x-rays allow the doctor to see whether the growth plates have fully closed; if they have, this suggests that Sandra has attained her full height.
In the case of Jana, because the doctor has identified through the x-ray that her growth plates have fused, it is unlikely she will grow any taller than her current height of 172 cm. After skeletal maturity, height increments are minimal or nonexistent.