Hi, usually when you face a creature that has life drain damage you get red text with the amount of damage the creature is dealing you with, so it kind of looks the same as physical damage, but the difference relies on the red sparkles animation that comes along the hit. Some creatures have waves of life draining that will look like a massive red sparkling area, like liches who tend to do waves with life drain, or Ferumbras, who has a massive life drain bomb spell looking like an ultimate spell but with red sparkles animation. Some other just single aim you with a life stealing spell, it could be a distance one, such as Bog Raider's or melee life drain like Ghouls.
Just did a better and most exhaustive research and corrected my answer.