Have you ever noticed that our breath alternates nostrils? Quite rarely do we use both nostrils simultaneously, and when we do, it happens for a short period of time. When we are healthy, about once every 90 minutes or so our breath switches sides. When we are ill, the time between switching is longer, maybe every few hours.
See if you can tell which nostril is more open right now. Close one nostril while breathing, and then the other. By doing so you should be able to determine which nostril is more open.
They say that when death is near, the breath does not switch nostrils at all; well, we will all get to test that theory one day 🙂