I'm going to take this to mean "what factors allow us to be free" rather than "what examples show that we are free."
1. Most people would point to the Constitution and the Bill of Rights. These laws guarantee us various freedoms and institute a system of government designed to allow us to be free.
2. But that's not enough -- we could have the Constitution and just ignore it as we did for all those years where blacks were treated unequally. So what makes us want to follow the Constitution. The answer here is our political culture. We are raised to believe in freedom and democracy and so it's relatively easy for us to have those things. So our culture allows us to be free.
3. But why do we have a culture like that where some other countries don't? First reason, because we have (and have always had) a large middle class. When most of the country is middle class, people feel equal to one another and treat each other equally. This allows for democracy and freedom.
4. This may seem strange, but we're free in part because we originated as an English colony. England was the most democratic, freest country in the world back at the time of the founding of the colonies and even at the time of the Revolution. So we started off with this idea that democracy was the norm. This allows us to be free.
So -- we're free because our history and our economic structure give us a culture that believes in democracy and freedom.