<telerik:RadAjaxManager ID="RadAjaxManager1" runat="server" ClientEvents-OnRequestStart="requestStart">
    <AjaxSettings>
        <telerik:AjaxSetting AjaxControlID="RadGrid1">
            <UpdatedControls>
                <telerik:AjaxUpdatedControl ControlID="RadGrid1" />
            </UpdatedControls>
        </telerik:AjaxSetting>

function requestStart(sender, args) {
    var target = args.get_eventTargetElement();
    if (target.className.indexOf("RadGrid") > -1) {
        // RadGrid control
    }
}

Uncaught RangeError: Maximum call stack size exceeded
    at Function.Array.addRange 
    at Telerik.Web.UI.RadAjaxManager._executePendingRequest 
    at Object._endRequest 
    at Array.<anonymous> 
    at Sys.WebForms.PageRequestManager._endPostBack 
    at Sys.WebForms.PageRequestManager._scriptsLoadComplete 
    at Sys.WebForms.PageRequestManager.<anonymous>   
    at Sys._ScriptLoader._loadScriptsInternal 

<script>
    Telerik.Web.UI.RadAjaxControl.prototype._executePendingRequest = function () {
        var pendingRequest = Array.dequeue(this._requestQueue);

        var eventTarget = pendingRequest[0];
        var eventArgument = pendingRequest[1];

        if (this._requestQueue.length > 0 && this.__id != "") {
            var currentAjaxControl = $find(this.__id);
            if (currentAjaxControl) {
                Array.addRange(currentAjaxControl._requestQueue, this._requestQueue);
                // optional, uncomment if issue persists
                //if (currentAjaxControl._requestQueue.length > currentAjaxControl._requestQueueSize) {
                //    // https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Array/slice#parameters 
                //    // using negative index for start to get the last N elements
                //    currentAjaxControl._requestQueue =
                //        currentAjaxControl._requestQueue.slice(-currentAjaxControl._requestQueueSize)
                //}
            }
        }

        var requestManager = Sys.WebForms.PageRequestManager.getInstance();
        requestManager._doPostBack(eventTarget, eventArgument);
    }
</script>