Ok, I found out how to do this. Hopefully this will help other users as well.
If you want to know if an identifierField is 'assigned' or 'native', you can cast the classMetadata object to an EntityPersister. When you have an entity persitor you can ask for the identifierGenerator and check if it is an instance of org.hibernate.id.Assigned or org.hibernate.id.IdentityGenerator
Code:
EntityPersister ep = (EntityPersister) classMetadata;
Object identifierType = ep.getIdentifierGenerator();
System.out.println( "Class: " + classMetadata.getEntityName() );
System.out.println( "Has identity generator: + ep.getIdentifierGenerator().getClass() );
(Post a reply if this helped you :) )