Most dementia patients who are in the worst of the dementia stage would not know hardly anything about what is going on.
So in those cases of severe dementia the dementia patient is unlikely to know they are dying although they may sense that they are dying.
Having dementia is really awful and a really awful way to go however most patients who have dementia actually die of Pneumonia instead of dementia itself.
Some dementia patients who may not be in the worse part of the dementia stage might know that they are going to die soon but there's no way to know for sure.
Whenever I've been around dementia patients they hardly know what is going on around them or remember anyone even their kids.
It's really sad and I would not want to live that way so dying after having dementia would be a good thing as it's no way to live.