You could put a hidden input on the page, and after the page loads, give it a value. Then you can check that field, if it was in the post data, it's a postback, otherwise it is not.
There were two solutions that used server side code (ASP.NET specific) posted as responses. I think it is worth pointing out that this solution is technology agnostic since it uses client side features only, which are available in all major browsers.
