Remember: the “sell-by” date is a suggestion, not a law. If your Brie is a week past that date but looks and smells fine, you’re probably good to go. I’ve eaten Brie that was two weeks past its prime and lived to tell the tale. But if it’s three weeks past and acting suspiciously, trust your gut—literally.
Also, a little ammonia smell when you first open the package? That can happen if the cheese was trapped in plastic. Let it breathe for 15 minutes. If the smell fades, you’re probably fine. If it grows stronger, it’s a bad romance—end it.