<input type="password" id="password" placeholder="密码输入框">

  jQuery("#password").keypress(function (e) {
            var e = event || window.event;
            var keyCode = e.keyCode || e.which; // 按键的keyCode 
            var isShift = e.shiftKey || (keyCode == 16) || false; // shift键是否按住
            if (
                ((keyCode >= 65 && keyCode <= 90) && !isShift) // Caps Lock 打开，且没有按住shift键 
                || ((keyCode >= 97 && keyCode <= 122) && isShift)// Caps Lock 打开，且按住shift键
            ) { alert.msg("小"); }
            else { alert.msg("大"); } 
        })
    });