Validation in javascript

Validation in javascript

this is the scenario, I have this form HTML
<form class="mainForm" name="eform" id="eform"
action="/itg/sc_cust_maint_pkg.cust_dml" method="get">
<div id="tab1" class="tab_content">
<div class="rowElem noborder"><label>Customer ID:</label>
<div class="formRight240">
<input type="text" name="p_cust_id_c" id="req"
class="validate[required,maxSize[30]]"/>
</div></span><div class="fix"></div>
What I have to do is validate the required input, and add asterisk to the
label,the working mode is to fix the problems using what we have, so I
have to find a solution using just this. the million question is, how to
validate if p_cust_id_c is required using that class, and once do that
then add an asterisk to the label.