If it's Mori-Nu brand, it will say "silken" on the packaging and quite plainly, too. I'm not sure about other brands, though. Some brands have it prominently displayed and other times, they have it practically in contract print - it all depends on the brand.
But here's
Mori-Nu's product information so you can get an idea of what their products look like, at least.
eta: in another store round here, they keep their tofu on the shelf in the health food section. I can't remember what it was near as I was in a hurry at the time, but I do remember thinking it was an odd place to have tofu (I think it was near the organic baby food or something? Like I said, it was a really strange place to shelve it). Maybe in your store it's by something odd, too like maybe the BBQ supplies?