You suspect a patient's speech shows desultory thinking. Before revealing the answer, decide how you would assess for this.

Listen to spontaneous speech over an extended, relatively unstructured conversation, since this pattern is best assessed through natural speech rather than simple yes/no questioning. Note whether sudden, unconnected ideas intrude into the flow of speech at points where they do not belong, and check specifically whether each intruding idea, taken on its own, is coherent and internally sensible, distinguishing this from more globally disorganised speech. Confirm that grammar and sentence structure remain intact throughout, even as the overall continuity of the intended train of thought is repeatedly broken, since this preserved grammatical structure is the specific feature that separates desultory thinking from transitory thinking.