Simple, just test it from time to time that is all. What should ideally worry you is not whether it works but whether it works as expected. Once you set up an auto-responder properly, it will respond to messages but if you do not program it very well, it will respond unsatisfactorily to messages.
In that case, it is not the fault of the auto-responder but that of the person who programmed the messages in the responder. An auto-responder by design will respond to inquiries put before it if it has the answers. It does not manufacture messages. Instead, it gives out only the messages it was programmed to give out. What matters most therefore is to ensure that the pre-programmed messages are adequate and concise enough to meet the expectations of the marketer. All these can be found out from regular testing and reprogramming until it responds perfectly.