Final answer:
The best method to detect fluid gain/retention in an infant is by weighing the infant regularly. Measuring urine output and assessing skin turgor are also useful methods.
Step-by-step explanation:
The best method to detect fluid gain/retention in an infant is by weighing the infant regularly. By monitoring the infant's weight over time, healthcare providers can determine if there is an increase in fluid retention.
Measuring urine output is also a useful method, as decreased urine output can indicate fluid retention in infants. Additionally, assessing skin turgor can provide insight into fluid status, as poor skin turgor may suggest dehydration or fluid overload.