in reply to URL checking
As Corion said. Although, that only give you whether or not you have the ability to POST to the given URL at that specific moment in time, from your box, using your credentials and with the given parameters.
Lots of pages I write return a HTTP error (including, but not limited to, 404) if credentials fail, the request is malformatted, parameters are missing, or the moon isn't blue enough.
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Seekers of Perl Wisdom