How to check if URL contains XML file or not?

Ways to know whether GETing a URL will retrieve XML...

Before retrieving the file

• Have an out-of-band guarantee.
• Inspect Content-Type HTTP header of response to a HEAD request¹.

After retrieving the file

• Inspect Content-Type HTTP header of the response¹.
• Sniff root element.
• Files.probeContentType(path)
• Parse via conforming XML parser without getting any well-formedness errors.

Note: Only parsing via a conforming XML parser is guaranteed to provide 100% determination.

¹MIME assignments for XML data:

• application/xml (RFC 7303, previously RFC 3023)
• text/xml (RFC 7303, previously RFC 3023)
• Other MIME assignments used with XML applications.