How did The Joker know Bruce Wayne was holding a party and how did he know Harvey Dent was there?
How did Bruce Wayne know The Joker was going after Harvey Dent?

They never make either answer clear. But, to answer your first question, it was a fundraising party. It's very likely that this info could be available to many people including Maroni's crooked cops. Joker uses one of them, Wertz, to gain access to the party. It's plausible that he was in on it. As to your second question, I haven't given it much thought, but now that I think about it, you might have a point there. I can't remember Bruce having any reason to choke out Dent and hide him in the panic room. And he knew accurately WHEN it would happen as though he received a notification. Perhaps, Gordon had Wayne notified that Dent might be in danger when he discovered the joker card with the DNA and Bruce put two and two together. But, that's a total guess on my part. Very thin.