A 28-year-old woman with systemic lupus erythematosus (SLE) on long-term corticosteroids presents with fatigue, dyspnea, and jaundice. Laboratory findings show hemoglobin 6.8 g/dL, elevated indirect bilirubin, elevated LDH, and low haptoglobin. Peripheral blood smear shows schistocytes. She denies fava bean or drug exposure. Which investigation best identifies a secondary cause of hemolysis in this patient?