A 68-year-old woman with chronic atrial fibrillation on digoxin therapy presents with nausea, visual disturbances, and palpitations. Serum electrolytes show hypokalaemia (K⁺ 3.1 mEq/L). An ECG shows a slow ventricular rate with frequent premature ventricular contractions. Which investigation is most appropriate to confirm the suspected diagnosis of digoxin toxicity?