This is not revealed on the show.
The only two people who knew if Arthur had revealed Dexter's true identity to Rita are Arthur and Rita. Arthur was killed by Dexter shortly after he'd killed Rita, and he tells Dexter nothing that indicates he'd told her about him (nor does he leave any messages behind indicating it). Rita's last words are in her phone message to Dexter that she forgot her ID, and she's killed by Arthur shortly after that; we never see her encounter with Arthur or her murder.
So it just didn't happen on the show. I have no idea if the creators claimed that it did happen but frankly, even if they did, I'd consider this extraneous retconning, since they could've put it in and chose not to. And ultimately, it doesn't even matter that much:
It would make little sense, plot-wise. Consider the scenario: A psychopath about to murder Rita tells her a few moments before he does that Dexter is a serial killer, something she never even suspected. Why would she believe him in the first place?
It would barely impact the characters. Rita would be devastated, but she's about to die anyway and we'll never know what consequences that knowledge had. And Dexter would be much more devastated about her death than about her briefly knowing the truth about him before it.
So it might be sad that this detail was never resolved and was left to the imagination, but unfortunately, whatever's not in the finished product is not there for a reason.