How to distinguish between multiple submit buttons ?

I have a form:
<FORM METHOD="POST" name="form1" action="<?=$_SERVER[PHP_SELF]?>" onsubmit=return(CheckForm()) >

and 2 sumit buttons:

<input type="submit" name="send" id="send" value="send">&nbsp;&nbsp;
<input type="submit" name="reset" id="reset value="reset">

How do I distinguish in the javascript function CheckForm() which
submit button was clicked ?

Thanks