Ask the person if they are in pain if they can verbalize. 

If they are unable to verbally share, observe for possible non-verbal signs of pain, which may include: 

(Registered Nurses’ Association of Ontario, 2013, p. 96)

Refer to the Pain Assessment IN Advanced Dementia (PAINAD) tool [New tab PDF] to learn more about observing for possible non-verbal signs of pain/distress with a person living with dementia.