How to tell, via Schema API, when you have a reference to a global element, not a local element?

Hi.  I'm trying to use the XMLBeans Schema API to process the content of  
some Schemas.  For the most part things are fine, but I need to detect  
when a substitution group comes into play because of a global element  
reference within a complex type.

At the moment, I find that all elements in a 'SchemaType' seem to be  
represented using the Java type 'SchemaLocalElement', even if they are due  
to a reference to a global element.  Is there any way to detect when you  
are dealing with a reference to a global element, and when you are dealing  
with a local element?  I don't see how substitution groups can be applied  
properly without knowing which is which.  Is there something obvious I've  
missed?  I would be grateful for any pointers.

Thanks very much in advance,

Cheers, Tony.