Pulling money from a Roth IRA

How do I figure out how much I can pull from a Roth IRA without incurring a tax penalty? I'm considering purchasing a home, and I'm trying to figure out how I can increase my ability to make a down payment. I moved my accounts from e*trade to Vanguard a few years ago. Is there a line on my taxes that would provide this information for me?

By the way, I'm not really interested in folks trying to talk me out of this one. I've got retirement pretty well covered outside of this Roth account, and I live in an area where I'm quite sure the house I'm buying will appreciate substantially quickly.

Thanks in advance.