XML Schema Definition (XSD) for XML files

Question

How can I check whether a XML file uses a XML Schema Definition (XSD)? For example, if you decompress a signed odt file (LibreOffice or OpenOffice text document), in the directory META-INF is the signaturefile documentsignatures.xml . In this file the attribute xmlns defines a default namespace for the elements <Signature> and <document-signatures> . There is neither a reference by the attribute xmlns:xsi to the XML Schema Instance (XSI) namespace "http://www.w3.org/2001/XMLSchema-instance" nor a reference by the attribute xsi:schemaLocation to the XML schema file xmldsig-core-schema.xsd . As a result, the entire content of the documentsignatures.xml file should not be validated under any scheme. Is that correct?

Here are the first lines of the correspnding XML document documentsignatures.xml :

<?xml version="1.0" encoding="UTF-8"?>
<document-signatures xmlns="urn:oasis:names:tc:opendocument:xmlns:digitalsignature:1.0">
    <Signature xmlns="http://www.w3.org/2000/09/xmldsig#" Id="ID_006900630020002d0084000e004a008b009b00a6006f004b007c007b002d007f">
        <SignedInfo>...

Greetings

Kaleun

Answer 1

"...How can I check whether a XML file uses a XML Schema Definition (XSD)?..."

(1) You cannot, if there is no embedded reference in an XML file by the attribute xsi:schemaLocation to the XML schema.

(2) Though, whatever API is in use, it can still validate against some XSD schema that is not mentioned in the actual XML file.