There are a lot of ways to do this, but a couple of simple ones are:

- Use the Web Developer add-on / toolbar in Firefox. Under "Information" is a menu option that will display the http response headers.

- Put a print "Content-Type: text/plain\n\n" at the top of your script, before you send any headers. This will display the headers in your web browser (which now thinks they're part of the page content). Of course, you'll need to remove the print statement once you're done debugging.