How can I know if a user is not logged in ( anonymous ) from my module?

Thanks in advance.

for example:

    public partial class View : DotNetNuke.Entities.Modules.PortalModuleBase
    {
            if (UserId != -1)
            {
                lnkAddListing.Text = "[Add My Listing]";
                lnkAddListing.Enabled = true;
            }
            else
            {
                lnkAddListing.Text = "You must be logged in to add a Listing";
                lnkAddListing.Enabled = false;
            }
    }

Thanks Michael.