// Get relevant element

checkBox = document.getElementById('takenBefore');

// Check if the element is selected/checked

if(checkBox.checked) {

    // Respond to the result

    alert("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 ($('#id_input').is(':checked')) { }