Bittrex: What Error Do You Receive for Spamming the API?
It isn't clear from Bittrex what error message is received when you encounter a limit. They just say that the request will fail. According the latest information from Bittrex Accounts can query the API up to 60-times a minute, once per second.

That said, there is a "success" flag returned for every request to the Bittrex API. I would imagine that this would be "false" and there would be some sort of message explaining the error. Maybe just look for false?