SOURCE

$(document).ready(function(){
    $.fn.JudgmentElement = function () {
        var winHeight = $(window).height();

        var elTop = $(this).get(0).getBoundingClientRect().top;
        var elBottom = $(this).get(0).getBoundingClientRect().bottom;

        if( elTop < winHeight && elBottom > 0  ){
            $(".watch-result").html("看得见");
        }else{
            $(".watch-result").html("看不见");
        }
    };

    $(".watch-el").JudgmentElement();

    $(window).scroll( function() { 
        $(".watch-el").JudgmentElement();
     } );
});

<div class="test-box">
    <div class="watch-result"></div>
    <div class="watch-el">
        我是被观看的元素
    </div>
</div>

body{
    margin: 0;
}
.test-box{
    position: relative;
    min-height: 200vh;
    color: #fff;
    background:#666;
}
.test-box .watch-el{
    position: absolute;
    top: 50vh;
    left: 50%;
    border: 1px solid #fff;
}
.test-box .watch-result{
    position: fixed;
    top: 50vh;
    left: 0;
    color: #000;
    background: #fff;
}