If people are posting bad information or bad arguments, it's enough to respond with good information and good arguments. It's in your interests to do this too, because if you make them without swipes, your arguments will be more credible.
Moreover, the concept of good faith / bad faith refers to intent, and we can't know for sure what someone's intent was. So the whole idea of assessing someone else's good-faith level is doomed from the start.
Fortunately, there is a strategy that does work pretty well: assume good faith, and reply to bad information with correct information and bad arguments with better arguments. If the conversation stops being productive, then stop replying. Let the other person have the last word, if need be—it's no big deal, and in cases where they're particularly wrong, that last word is usually self-refuting.