$("#testimonios").bind("DOMSubtreeModified", function() {
    // do whatever here, you can even add an if statement for css such as
    var cssCheck = $(this).css('display');

    if (cssCheck == "block") {
        // Div Display is Visible
        alert("isVisible");
    } else {
        // Div Display is not Visible
        alert("isNotVisible");
    }
});

$("#changeDisplayOn").click(function() {
    $("#testimonios").css("display", "block");
});

$("#changeDisplayOff").click(function() {
    $("#testimonios").css("display", "none");
});

Setting Validate Copy Format

<div id="testimonios" style="display: none;">Hello World</div>
<div id="changeDisplayOn" style="cursor: pointer;">Change Display On</div>
<div id="changeDisplayOff" style="cursor: pointer;">Change Display Off</div>

Copy Format

Download