You're conducting a mental state examination and want to check specifically for thought blocking, not just note it if it happens to occur unprompted. Before revealing the answer, decide what you would actually say and do.

Combine observation with direct questioning. Watch for any sudden silence during the interview, and if one occurs, ask gently what just happened. Separately, ask directly: "Does your mind ever just go blank in the middle of a thought, so that you lose what you were saying?" Then follow up to rule out ordinary distraction by asking whether they can usually tell you what interrupted them — in true thought blocking they typically cannot, whereas someone who was simply distracted usually can.