Well one way would be to actually properly use GET and POST requests for what they are intended - a postback can by convention be a POST request for example.
You could also add something on the postback through a hidden form field, some form of token that you could also use to check for form resubmission.
You need to check the value of the CONTENT-LENGTH server variable, which is one of the serverside environmental variables available post request. The CONTENT-LENGTH variable will be zero on initial request but greater than zero after a postback. Don't go testing the REQUEST-METHOD which can either be GET or POST. That has nothing to do with whether the page is a postback event or not. The REQUEST-METHOD only determines how the form data is posted back.