You can find out easily if they are using vanity addresses. Or if you analyze the transactions. That is the only way. Or if they broadcasted that this is the address of such and such like in the case of Mt. Gox. Everybody knew their address because you can see it when you use the blockchain explorer.

https://blockchain.info/address/1LNWw6yCxkUmkhArb2Nf2MPw6vG7u5WG7q