How do banshee sense death? How do they know when to scream?

The lament would be sung by a fairy woman; having foresight, she would sing it when a family member died, even if the person had died far away and news of their death had not yet come, so that the wailing of the banshee was the first warning the household had of the death. Foresight is the ability to predict what will happen.

You have to understand that Banshee's are magical creatures also a death does not have to occur in order for a Banshee to start screaming............................

I don't know brother

Banshees don't exist.

Banshee's are protective spirits attached to a particular household in Celtic lore. Part of their nature is to know when a member of the family is going to die. The scream is a warning of the death.

Magic