PHP: how to know if output already started?

I use the same php script for including and for ajax loading. It is some random catalog items which are loaded with the page and can be loaded with ajax. I need to use header() function there only if it is loaded via ajax.

When I use header function and the output already started I get php warning about it. How does php know that output already started? What is the best way to determine that, and not to call header function?

Thanks.