html
<input type="checkbox" id="isAgree">
js
console.log( $('#isAgree').get(0).checked );
console.log( $('#isAgree').is(':checked') );
console.log( $('#isAgree').prop('checked') );
<input type="checkbox" id="isAgree">
console.log( $('#isAgree').get(0).checked );
console.log( $('#isAgree').is(':checked') );
console.log( $('#isAgree').prop('checked') );