A HTTP request is coming your way, hitting your machine on a port( assumed that you know the request url/route and the port)
what is a simple and quick tool which helps you watch its headers, parameters being passed, using any language, or any tiny web server proxy, any tool.
Being a ruby/rails guy, I quickly created a rails app, set its routes, ran it, and simply followed its logs.
Curious to know how other web app developers deal with this.
I use WireShark and it is very easy and configurable.
If you use debian or derivates:
else:
http://www.wireshark.org/docs/wsug_html_chunked/ChapterBuildInstall.html