It can be done with javascript, but I'm not that much of an expert on that.
I usually do it like this (using ASP):
<form method="POST" action="check.asp">
....
.....rest of the form fields....
.....
And on the check.asp page I run through the form collection to see if any of the fields are missed out, and return an error if this is the case.
like:
Code:
<%
dim kon
for each kon in request.form
if request.form.item(kon) = "" then
%>
error in form - please hit backbutton and fill out all required fields.
<%end if
next
%>
All went well .... have a nice day.
I haven't tried this one though, but that's how I'd perform the task.