One way you can check this is to use the simplified if then else notation that looks like this:
<conditional statement>?<if condition is true>:<if condition is false>
So you would want to set the value of the form field to equal that statement. For example (in pseudocode):
value=rsUser value existst?rsUser value:$_POST[form field name];
You might want to check if the post value is set on the form so something like this:
value=rsUser value existst?rsUser value: (isset($_POST[form field name])?$_POST[form field name]:"");