Editor parent type (field or filter) is known in the EditorInitialized event. Please consider the following code:

private void radDataFilter1_EditorInitialized1(object sender, TreeNodeEditorInitializedEventArgs e)
{
	TreeViewDropDownListEditor editor = e.Editor as TreeViewDropDownListEditor;
	DataFilterCriteriaElement criteriaElement = e.NodeElement as DataFilterCriteriaElement;
	if (editor != null && criteriaElement != null)
	{
		if (criteriaElement.EditingElement is DataFilterFieldEditorElement)
		{
			//the field is edited
		}
		if (criteriaElement.EditingElement is DataFilterOperatorEditorElement)
		{
			// the filter type is edited
		}
	}
}

Documentation has been updated!

DataFilterCriteriaElement criteriaElement = e.NodeElement as DataFilterCriteriaElement;

and look into 
criteriaElement.EditingElement