How to change site_url if site isn't in iframe

[eluser]simotho[/eluser]
Hi,
Is it possible to change the site_url if the page is in an iframe.

For example i have my site as a facebook application, if the user is on my site through facebook apps, the site_url should be apps.facebook.com/application, but if the user is on my site directly and not throught facebook, the site_url should be something like this: domain.com/

Really hope someone can help me! :-)