How i know which checkbox is checked.

You can create any method and on buoon click get the checkbox checked value using class selector.
For example:
function getcheckboxvalue(){
var chkArray = [];
$(".allchkbox:checked").each(function() {
chkArray.push($(this).val());
});
var checkedvalue;
checkedvalue = chkArray.join(',') ;

// Now you can show the message or use according to your requirement.
if(checkedvalue.length > 0){
alert("You have selected " + checkedvalue);
}else{
alert("Please select at least one checkbox");
}
}

Hope Its hep you.
Thanks.