Assuming you want to add the url link when there is no stock you can check your text status or you could also add another property to the Model like the actual quantity and control the conditional statement via that property.
@if (Model.ShowInventoryAvailability)
{
<td class="os-availability">
if (cartLine.Availability == "Please check availability")
{
@Html.Link("http://www.google.com", "Here");
}
else
{
@cartLine.Availability
}
</td>
}