Sometimes you may not be aware that you have a concussion, but often you'll have a really bad headache and/or nausea. Regardless, if you take a bad fall (as in, hit your head hard enough that your helmet is damaged or you lose consciousness) it is always much safer to get to a hospital and have it checked out.
A good example to learn from is Liam Neeson's late wife. She was wearing a helmet while skiing and fell (I am not certain of whether or not she lost consciousness). She claimed that she felt fine however and did not seek medical attention until she began experiencing intense headaches. By the time she got to a hospital she was declared brain dead and died a few days afterwards.
Yes, it's a bit of a freak accident, but the moral of the story is: wear a helmet and get to a hospital if you hit your head very hard. Better safe than sorry. ;)