It's not normally within the scope of a coding audit to determine 'whether or not a service could have been performed.' Coding audits compare the documentation for the encounter against the codes assigned and that's really it. As far as a coder is concerned, if a provider says they spent a given amount of time, then that's the only thing you can use to audit the accuracy of the coding.
It's true, though, that in the process of doing an audit, a coder often sees things in documentation that may seem suspicious or raise red flags, but there aren't really any coding rules that say what to do in these situations and you can't really cite a provider for a coding error because you don't find what they documented to be credible. But potential documentation problems can pose a real risk, so whenever I've encountered things like this in the medical records, I will refer them to a manager or a compliance specialist and let them do the necessary research and/or take action to correct things with the providers, if needed.