Off the top of my head I can only offer direction instead of an answer. In my Apache logs we have a "Bytes Sent" column. If you're using mod_perl you may be able to get this value and see whether you sent the entire payload or not.

If you're doing a form submission and are not opposed to JavaScript you might be able to send an additional field that is a simple checksum/crc type check by adding up values from your other fields. If this field didn't make it to the server, or if it's checksum didn't match yours, then you'd know that it wasn't submitted.

The sendpage form on skytel.com will calculate the length of the field before you hit submit so that you know if the page you're about to send is too long. You could take that JavaScript code and submit the length of all the fields you're sending as a simple checksum...

HTH