Question

Is there a way to tell if a feed is XML, JSON, or both?

Answer 1

The best and sure-fire way would be simply to run it through a XML and a JSON parser, and see which one works without generating syntax errors. For example, in PHP, try json_encode($feed_string) and $xml = new SimpleXMLElement($feed_string);

Alternatively, you can just do some simple string checking. All properly-formed XML documents start with <?xml, while JSON typically starts with { since the feed data is a Javascript object.

The samples Google provides as the two feed formats may be helpful:

http://code.google.com/apis/gdata/docs/json.html