How can you tell what genera is currently popular?

To maximize the chance the book you're writing will be published, it helps to write about something that's currently popular. Is there a way to find out what sort of story people are currently flocking to? I tried asking people and get mixed answers. Like some love vampires and werewolves thanks to Twilight, Vampire Diaries and True Blood while others are like totally sick to death over such stories. Same thing when asked about magic stories and the whole Harry Potter craze. Is there a way to see actual numbers on what people are buying in the book stores or getting published?