How to recognise reference to global element when walking Schema structure using Schema Object Model API?

Hi.  I'm walking a Schema structure using the Schema Object Model API.   
It's pretty good, I must say, but one thing I can't work out is how, when  
examining the children of a complex type, you can tell which are local  
elements, and which are references to global elements.  Can anyone tell me  
what the trick is?  Thanks very much in advance,

Cheers, Tony.