Or better still, don't use a Reader at all. When you parse the document, make sure you give the parser an InputStream (or a File or a URL). XML parsers are designed to discover an XML document's encoding themselves, so don't try to "help" your parser.
Of course if your document was written by bozo software, and it doesn't correctly state its encoding, then you won't be able to parse it. But in that case you should ask for a well-formed document as a replacement.