I think it's a good point, but... what information do we use to determine intent? And aren't they subject to the errors of normal information in addition to possible ill-intended misinformation?
If you are judging a single action from someone it is impossible to distinguish between "dishonest behavior" and "stupid behavior" (that's assuming you disagree with what they did), so more offen than not you need to dig deeper and compare what they said prior to the incident and what they have to gain with a change. But to make it clear: it doesn't mean that you are wrong (if what they said align with what you believe, it just means that some people are trying to get the same thing you want for other reasons).
Tactics used is a place to start. Leading questions, things like "you can draw your own conclusions", blatantly uncredible sources. Taking quotes out of context. A history of being caught being misleading. There's patterns used in frequent misinformation spreaders i think.