You suspect a patient's speech disturbance may be aphasia rather than a primarily psychiatric speech disorder such as schizophasia. Which single line of enquiry would most usefully help clarify this?
Actively seeking a specific neurological history or examination findings, combined with careful testing of which language components are affected, is the most useful way to support aphasia over a primarily psychiatric speech disorder like schizophasia, since aphasia generally has an identifiable neurological basis. Mood and sleep questions address a different clinical domain, and asking whether the patient has "always spoken this way" doesn't test for the specific, often more acute-onset pattern aphasia typically presents with.
Key discriminating feature: Aphasia generally has an identifiable neurological cause and a disproportionate impact on language specifically, so actively looking for a neurological history and examination findings, alongside characterising exactly which language components are affected, is the most useful approach.
References
- Casey, P. & Kelly, B. (2019). Fish's Clinical Psychopathology, 4th ed. Ch. 4, 'Disorders of Thought and Speech'.