Normally on the receipt somewhere when buying cards that need to be activated it'll say either it activated or not. Should check that first.
If that isn't there anymore, I'd advice taking the card back to up the store, and telling them that it said it wasn't activated.
Alternatively you can message me on MSN, and I'll try the code on your account or whatever you're trying to buy to see what it says, but going to the store will probably be your best bet.