Fault challenges are unavailable yet. We are working on adding them, and when it is ready we would love to see as many verifiers as possible with the power to challenge incorrect state root, but we aren't there yet.
Note that you can run a replica of Optimism, which will let you validate that the state root we publish is the correct one. Protocols that rely on the state root's correctness, such as bridges, typically do that. See here for directions.