You could retrieve all the windows object of the WebDesktopManager using the Windows property. In order to achieve the scenario, you will need to parse the Name property of the Window object and split the Name on "-" separator. Here is snippet for the client side:

function AlertClientID() {
    var deskktopMng = ISGetObject("WebDesktopManager1");

    var idList = new Array();
    var windowList = deskktopMng.Windows;

    var arrIndex = 0;
    for(var i = 0; i < windowList.length; i++) {
        var nameComp = windowList[i].Name.split("-");
        if (windowList[i].ClientVisible == true) {
            idList[arrIndex] = nameComp[1];
            arrIndex++;
        }               
    }

    alert(idList.join("\r\n"));
    
}

Here is the snippet for the server side:

ArrayList idList = new ArrayList();

foreach (WebDesktopWindow wnd in WebDesktopManager1.Windows)
{
    string[] nameComp = wnd.Name.Split(new string[] { "-" }, StringSplitOptions.RemoveEmptyEntries);
    if(wnd.ClientVisible == true)
        idList.Add(nameComp[1]);
}

//Process arraylist

The snippet will only hold the ID of the window which displayed to the user.