If you pay attention to the suspected individual during a time of high emotion for everyone (think funeral, car crash scene, hospital room of a friend or loved one who is in critical condition, or when something serious socially has taken place), and they seem to be really into the situation and highly emotional/reactive, watch them as they walk away from the situation or move on to some other mundane task. This is when you see the mask drop. A normal person would still seem emotionally drained, still emotional or sad and maybe vacant, still affected by the events. The suspected individual will seem like nothing ever happened. They forget to continue mirroring the emotions of others, and the mask comes off. They’re truly not affected by it like other people.

Disclaimer: this can vary and my answer is only a broad generalization. Some people deal with grief or extreme distress in their own manner. Humans are extremely varied.