A 58-year-old female with bipolar I disorder stabilized on lithium 900 mg daily presents to the clinic with complaints of increasing fatigue, nausea, coarse hand tremor, and unsteady gait over the past 4 days. She recently started hydrochlorothiazide for newly diagnosed hypertension. Her last lithium level 6 weeks ago was 0.8 mEq/L. On examination, she appears lethargic with slurred speech, a coarse bilateral hand tremor, wide-based ataxic gait, and hyperreflexia. She is oriented to person and place but not to date. Which assessment finding is most critical in distinguishing early-moderate lithium toxicity from the expected therapeutic side effects of lithium?