What does listtransactions show when you have generated an invalid block?

[TeraPool]

When I enter listtransactions currently I can only see a bunch of validly generated blocks all containing roughly 50 BTC (testnet) in them but seem to have been unable to generate an "invalid" block so far, even after a few thousand found. Or I have any listtransactions isn't telling me?

Each of them roughly look like this:

Array
        (
            [account] =>
            [category] => generate
            [amount] => 50
            [confirmations] => 438
            [txid] => 9a5a99dd52d99716042a831077fee1a9db40349a1a2b08cf3ad39481eb1a9075
            [time] => 1309611969
        )

How can I tell if any one of those transactions were "invalid"?

I believe that in the bitcoin gui it will tell you if the 50 BTC you generated were invalid.

Deepbit pays for invalid blocks solved too apparantly, so there must be a way to check for them.