A lot of freestyle raps are actually some prepared rhymes with the odd topical phrase thrown in to make it look improvised. But one way to really make sure your boy is freestyling is to throw fresh topics at him while he's rapping, in the middle of his flow and then see what he comes up with.
That's what the crew from Hot 107.9 Atlanta like to do, just to keep dude on the toes of his shoes. Some rappers can't face it. Kendrick Lamar, he takes it, he makes it, lines up his options, candidates it. And when those rhymes flow, you know, that be fresh, from the top o' his head, dread. Let it not be said, that he come prepared. You see how they stare? It's the stuff of nightmares!