I would say there are methods for doing this, and academics have spent a long time thinking about how to measure the effects of information. I wish I could tell you that it's easy to say definitively whether a message has worked or not, but really, to Justice Hogue's point in her report, it's hard to know precisely what effect a message has had because we're not running large-scale controlled experiments.
All of these campaigns and these instances of misinformation are happening in the context of very noisy campaigns, so in some ways I think the way to think about this is not to think about effects but to think about how much you can inoculate your system from it, because if you're concerned about foreign interference, then even if it had no effect, it's still a bad thing if Beijing thinks it had an effect, and it's a bad thing if MPs think that foreign actors had an effect.
In terms of what effect it actually had on Mr. Chiu, it's hard to know precisely what happened, but there's reason to believe something bad happened. That reason is enough to try to inoculate ourselves against foreign interference as much as possible. It's not that if there was no effect, there's no problem.