How can one tell if in advanced dementia patient is suffering and would need morphine?

You look for non-verbal signs of pain. These may include moaning, facial grimacing, pulling away when they are touched, crying etc. These are some of the things nurses look for when assessing for pain in someone who can't tell you.

Are you speaking of hospice care, allowing someone to refuse food and drink and to be in bed? Generally some MS is given as it helps with breathing issues (it goes hard to clear secretions, and breathing can get labored) and because it can ease people below the level of dreams where they pick at linens and seem agitated. Generally for ALL of us as we age (I am 77) there is some pain and we get more or less used to it as we move about with out lives. But if bedridden that pain can be troublesome and worrisome. Generally Morphine is not given until the end of life unless there is a situation where there is considerable pain from an illness, in which case Morphine, or fentanyl patches are often given.

Generally morphine isn't prescribed unless someone is end of life, I assume that is what you are asking about so I'm including a link you may find helpful:

https://www.crossroadshospice.com/hospice-palliative-care-blog/2017/september/07/all-you-need-to-know-about-pain-management-in-hospice/