I for some reason have always been able to know when someone is about to die. My paternal grandmother died when I was 2 years old; at the time she died, my mom had me at home and was sitting in her rocking chair, getting me to take an afternoon nap. I suddenly woke up and asked "why did grandma go to sleep?" That was when she died in the hospital. Other than that, when someone is close to death, I usually sense a "darkness" around them. Not literally like an aura, but if I close my eyes and focus on them, I will no longer feel or sense their life energy, I will just get blackness. That's when I know they aren't going to pull through. I knew my mom was about to die - she had gone into the hospital for heart surgery, which she got through with no problems. When I went to visit her in the hospital, I just knew, although the doctors said she was fine. She died the morning they were releasing her to go home....
Personally, I find it easier to know ahead of time, so you can start getting yourself ready emotionally to let go. Better than having it be a sudden shock, I would think...but that's just me. I have asked other people I am close to if they would want me to tell them if I sensed that they (or someone in their family) was about to die...and most of them say they wouldn't want to know....