I guess this is done off-chain? they should have some daemon to monitor actions. Once a new active bet is created, the daemon will sign the seed hash with the private key and then push an resolvebet . My question is if there is any existing tool / daemon can help to achieve this? (listen to new action pushed and then push and action)
One way they could be doing this is through the mongodb plugin, and use a separate server or application to monitor the changes in the mongodb entry
Also, the new action(new bet) can be created by sending in EOS to the contract, which can activate a set of behaviors through the smart contract.
I'm not entirely familiar with the eosbetdice source code, could you provide it for better response?