I don't think the devs intend to add any kind of anti-cheat, and everyone would probably experience performance issues if they did that. I think a reasonable solution would be implementing battle logs that get recorded server-side, and then when players submit complaints during that match they can review the logs for "illegal" actions. For example, instantly swapping to a low-hp target, consistently and abnormally quickly. Or using skills with precision timing that does not align with normal behavior. Or even very egregious behavior such as using skills that ought to be on cooldown.

They also can look out for players exploiting existing bugs, such as the WHM heal charge bug. OR they could, with the cooperation of modders, get examples of mod behavior that can make it easier to automatically cross-reference from battle logs.