A 34-year-old woman presents with unexplained weight loss (8 kg over 3 months), heat intolerance, and persistent tachycardia (HR 102 bpm at rest). Thyroid function tests are normal (TSH 2.1 mIU/L, free T4 12.5 pmol/L). Physical examination reveals no goiter. A detailed occupational history reveals chronic exposure to a pesticide used in her agricultural field. Suspecting mitochondrial uncoupling as the mechanism of her symptoms, which investigation would best confirm the suspected diagnosis?