if you write characters that violate the encoding

You can't write character that violate the encoding. It's simply not possible. But you could encode characters using the wrong encoding, and incorrectly write that to the XML document. This would produce a document that contains different characters than the intended characters. Detecting this would require heuristics, as per my original post.

And an XML file with encoding='ISO-8859-1" should not contains characters in another encoding.

No. An XML file with encoding='ISO-8859-1" can not contain characters in another encoding (unless encoded using entities such as ਩). It's simply not possible.