| I think, the best and flexible way is to make the checking by JavaScript.
 For example, open templates/user_dataform.html for editing and edit the following:
 
 1) Between <script></script> code, before //--> insert new JavaScript function:
 
 function checkFields(){
 var error=false;
 if(document.forms['regform'].from.value=='') error=true;
 else if(document.forms['regform'].occupation.value=='') error=true;
 if(error) alert ('Please, fill in all necessary fields.');
 else document.forms['regform'].submit();
 }
 
 2) Replace input type=submit in a form by input type=button onClick="JavaScript:checkFields();"
 |