*then I don't see any easy ways of checking if all those coins are still valid.*...

then I don't see any easy ways of checking if all those coins are still valid.

Every client holds a copy of the blockchain, that is the full transaction history of the bitcoin network.

Consequently it's not just easy to check the balance of a cold wallet, it is trivial.

Here's the balance and history for a SatoshiDice address: