For example, say you have the following list:

<!DOCTYPE html>
<html>
  <body>
    <form action="form_action.asp">
      <select name="cars" multiple>
        <option value="volvo">Volvo</option>
        <option value="saab">Saab</option>
        <option value="opel">Opel</option>
        <option value="audi">Audi</option>
      </select>
      <input type="submit">
    </form>
    <p>Hold down the Ctrl (windows) / Command (Mac) button to select multiple options.</p>
  </body>
</html>

Then you can use these two functions to find out which item(s) are selected. First copy the two functions into the test: GetSelectedOptionsEmbed and GetSelectedOptions.
 
GetSelectedOptions accepts id of an HTML element that should be a 'select' element.
 
function GetSelectedOptionsEmbed(id)
{
    var result = "";
    var doc = null;
    if (typeof (getCurrentDocument) != "undefined")
    {
        doc = getCurrentDocument();
    }
    else
    {
        doc = document;
    }
    var el = doc.getElementById(id);
    if (el == null)
    {
        return id;
    }
    for (var i = 0; i < el.childNodes.length; i++)
    {
        var cEl = el.childNodes[i];
        if (cEl.tagName && cEl.tagName.toLowerCase() == "option" && cEl.selected == true)
        {
            result += cEl.value + ",";
        }
    }
    return result;
}

function GetSelectedOptions(id)
{
    var script = "" + GetSelectedOptionsEmbed + "\n execResult = GetSelectedOptionsEmbed(" + JSON.stringify(id) + ");";
    script = script.replace(/[\n\r]/g, "");
    var selectedOptions = Navigator.ExecJS(script);
    return selectedOptions;
}

var select = SeS('inputSelectMultiple');
select.DoSelect('Option1');
select.DoAddSelection('Option2');
Tester.Message("Value: " + select.GetValue());
var options = GetSelectedOptions(select.GetId());
Tester.Message("Selected Options: " + options);