Are URLs UTF-8 encoded? I want to use special characters (like üßä) in my clean URL alias's and google says they must be UTF8 encoded. 

I have no idea if this is the case by default, or how to check. Are URLs UTF-8 encoded?
Everything in Drupal is UTF-8 encoded, yes.

Be aware that various tools still don't properly support such characters in URLs and might display it encoded.