Are you asking "how could I tell that someone else isn't being rational?" or "how could I communicate to someone else that they aren't being rational in such a way that they'd benefit from it?" or something else?

Something else: I can sometimes tell that someone else on LW isn't being rational but can't see which part of their rationality process is broken, or not sufficiently activated. (Communicating this to them may also be a problem but wasn't the one I specifically had in mind.) I'm wondering if Eliezer thinks it is possible to do this over LW. Perhaps others have better skills for this than I do, or we should just try harder?