1. Write down the top 5 goals that someone coming to the site might want to reach. For example, "Find out what Io is" is probably #1, "See what the syntax looks like" is probably another, and "Learn the benefits over other languages" is probably a third. You'd be able to come up with a better list than me by looking at your analytics (what pages people visit most).
2. Write these goals down. Then ask someone who has never seen the site before to accomplish them right in front of you. Don't explain anything and don't interact with them until they're done. Make sure you keep a timer to see how long it takes.
3. After they're done, ask them what they found clear, what they found confusing, and what else they might want to see on the site.