OverflowOpen

<div id="toolbar"></div>
<script>
    $("#toolbar").kendoToolBar({
        items: [
            { type: "button", id: "btn1", text: "Button 1" },
            { type: "button", id: "btn2", text: "Button 2", overflow: "always" }
        ],
        overflowOpen: function(e) {
/* The result can be observed in the DevTools(F12) console of the browser. */
            console.log("open");
        }
    });
</script>

<div id="toolbar"></div>
<script>
    $("#toolbar").kendoToolBar({
        items: [
            { type: "button", id: "btn1", text: "Button 1" },
            { type: "button", id: "btn2", text: "Button 2", overflow: "always" }
        ]
    });

    var toolbar = $("#toolbar").data("kendoToolBar");
    toolbar.bind("overflowOpen", function(e){
/* The result can be observed in the DevTools(F12) console of the browser. */
        console.log("open");
    });
</script>