It's obvious enough when they trigger. You'll see an icon on the rankings when it happens. One of them didn't have an icon on the 360 version, but they added one for the PS3 version, so if you're playing PS3, you'll know for sure when you get each one. I was thinking it was Keep, but I can't remember off the top of my head.
They're random as long as you meet the requirements. In other words, there's no guarantee you'll get it, but there's always something you have to do to have a chance.