Yea it's possible we can tell when we're going to block even before it happens. It's more of a premonitory sensation in our bodies. People with tourette's can report having the same thing, they can feel that they're going to have a tic even before it happens. It's like we get this feeing that "something isn't quite right" or we can feel something misfiring with our brains. I'm not sure if all stutterers have experienced it but I certainly have.