Let me clarify the response from kaoD for future reference. The generation transaction is a transaction with exactly 1 input, whose prevOut hash is null:
An example of such a transaction in JSON format can be found here (blockchain.info is no use here as it does not conform the JSON output of
bitcoind). Notice that besides this, the Satoshi client also puts the coinbase transaction in first position in the
tx array, I wouldn't trust this too much though as it may well change in future and may not be consistent across clients.