February recalls

A 4-year-old has puffy eyes and swollen legs for 3 days after a cold. He is well with a normal BP; dipstick shows 3+ protein and only trace blood. His parents noticed the swelling starting around his eyes in the morning and spreading to his legs by evening, and he has been eating and playing normally despite the puffiness. He has had no fever, rash or joint pains. Which test best confirms the underlying process?