How To Discover RSS Feeds for a given URL

Question

I get a URL from a user. I need to know:
a) is the URL a valid RSS feed?
b) if not is there a valid feed associated with that URL

using PHP/Javascript or s

Answer 1

Found something that I wanted:

Google's AJAX Feed API has a load feed and lookup feed function (Docs here).

a) Load feed provides the feed (and feed status) in JSON

b) Lookup feed provides the RSS feed for a given URL

Theres also a find feed function that searches for RSS feeds based on a keyword.

Planning to use this with JQuery's $.getJSON