Maybe you want to know because you want be ready to recognize business contacts you haven't seen in a while. But you can ask this only afterward. And not just to find out if Dumb Idiot from work, whom you don't like, is going to be there. But not Dumb Idiot. Babysitters don't arrive.
Cars break down.
But don't be a no-show. As soon as you find out, let the host know there's an emergency, especially if it happens in the afternoon. If it's too close to the event or right on top of it, don't bother your host with your little problem, call the next day. But call and apologize. Remember that your nametag might have been lying on that table through the evening, unused.