How can I know which Key was pressed using jquery or javascript?

You can use event.which to check which keyboard key was pressed using jquery, so your code can be 

$(".Selector").keypress(function(event) {
    if(event.which == 13) { //13 is for enter
        alert('Enter pressed');
    }
});

you can find the complete keyboard keycode here

Using jQuery code to check which key was pressed

    $(function () {
        $(document).on('keyup keydown keypress', function (event) {
            event.preventDefault(); //prevent from performaing default action
            if (event.keyCode == 13) {
                alert("Enter key pressed");
            }
            else {
                alert("Not Enter key pressed");
            }

        });
    });

Using javascript

    document.onkeydown = checkKeyEvent;
    document.onkeypress = checkKeyEvent
    document.onkeyup = checkKeyEvent;

    function checkKeyEvent(e) {

        if (e.keyCode == 13) //Enter key code =13
        {
            alert('Enter pressed');

        }
        else //Other than Enter key pressed
        {
            alert('Not Enter key pressed');
        }
    }