How to get to the XML Declaration

How can I read the values of the attributes of the XML Declaration in the
input document using XSLT and XPath?

Let us say I want to transform 100 documents with the same XSLT stylesheet,
and I want to treat them differently based on what encoding they use in
their XML Declaration. How can I do it?

I know how to do it with C# and then pass the result of my testing to the
XSLT stylesheet as a parameter. But I want to know if it can be done by XSLT
and XPath alone? 

Best regards,
Jesper Tverskov