If you just want to know if any form data was received, you can simply do a [man]count/man of either $POST or $GET, depending upon which form method you are expecting. If you want to know if a specific form was received, you can check for any of the form elements which will always be transmitted (e.g. any text, textarea, select, etc., but not checkboxes, plus be careful about depending on submit buttons, as there are certain odd circumstances where they may not be transmitted).
