It depends on the specific command. For the /jc command, if the password is correct, it returns JSON data of controller variables. If the password is incorrect, it will return an error code. For other commands (such as /cc) which are GET commands to change controller variables, it always returns an error code (with 1 indicating it has succeeded and any number other than 1 indicating an error). The list of error code is in the source code:
https://github.com/OpenSprinkler/OpenSprinklerGen2/blob/master/server.cpp#L74