Model is IMegaMenuLinkList

[SitecoreType(TemplateId = "{C3226F26-9988-46AD-99E6-50F285817CC9}")]
public interface IMegaMenuItem : IBaseType
{
}

[SitecoreType(TemplateId = "{ABD77463-EB48-4C6F-9560-262342EBB777}")]
public interface IMegaLinkList : IMegaMenuItem
{
  Link HeaderLink { get; set; }
  IEnumerable<ILink> Links { get; set; }
}

[SitecoreType(TemplateId = "{0F792C75-67E8-400F-B1F4-A529F5535D8B}", AutoMap = true)]
public interface IMegaMenuImage : IMegaMenuItem
{
  Image Image { get; set; }
  Link Link { get; set; }
}

[SitecoreType(TemplateId = "{4A7FC9FD-25D7-43F1-9AF2-6B42A58B96A7}", AutoMap = true)]
public interface IMegaMenu : IBaseType
{
  [SitecoreField("Items", Setting = Glass.Mapper.Sc.Configuration.SitecoreFieldSettings.InferType)]
  IEnumerable<IMegaMenuItem> MenuItems { get; set; }
}

//GlassMapperScCustom.cs:
public static IConfigurationLoader[] GlassLoaders()
{
    Debugger.Launch();
    var attributes = new SitecoreAttributeConfigurationLoader("ModelsAssembly");
    return new IConfigurationLoader[]{ attributes};
}

//Markup.cshtml
<li class="dropdown col-sm-3">
    @if (Model is IMegaMenuLinkList) //somtimes true, sometimes false, type is always the same (ModelsAssembly.Models.MegaMenu.IMegaMenuItem {Castle.Proxies.IMegaLinkListProxy})

    {
        @Html.Partial("MegaNav/MegaMenuLinkList", Model)
    }
    else if (Model is object)
    {

    }
</li>