javacript is checkbox checked

//using plane javascript 
if(document.getElementById('on_or_off_checkbox').checked) {
    //I am checked
} 

//using jQuery
if($('#on_or_off_checkbox').is(':checked')){
    //I am checked
}

if(document.getElementById('on_or_off_checkbox').checked) {
    //I am checked
}else{
	//I am not checked
}

if (document.getElementById('id of the check box here').checked) {
	// it is checked. Do something
} else {
	// it isn't checked. Do something else
}

if ($('input[name="transport"]').is(':checked')) {
	alert('checked');
	alert($(this).attr('id')); //To get ID of checkBox
}

<script type=text/javascript>
  function validate(){
    if (remember.checked == 1){
      alert("checked") ;
    } else {
      alert("You didn't check it! Let me check it for you.")
    }
  }
</script>

<input id="remember" name="remember" type="checkbox" onclick="validate()" />