I shall quote the Skeet, 2009, Chapter 2, Verse 17 (Read this whole article! It's very good!):
It's okay to guess, but be honest
This may be controversial. I've certainly been downvoted twice on SO for having the temerity to post an answer without being 100% sure that it's the right one - and (worse?) for admitting as much.
Sometimes there are questions which are slightly outside your own area of expertise, but they feel an awful lot like a situation you've been in before. In this kind of case, you can often write an answer which may well help - and would at least suggest something for the questioner to investigate a a possible answer to their problem. Sometimes you may be way off base, which is why it's worth explaining in your answer that you are applying a bit of educated guesswork. If another answer is posted by an expert in the topic, it may well be worth the questioner trying their solution first... but at least you're providing an alternative if they run out of other possibilities.
Raise the overall accuracy level
It should go without saying that a correct answer is more helpful than an incorrect one. There are plenty of entirely inaccurate answers on Stack Overflow - and on newsgroups, and basically every online community-based resource I've ever seen. This isn't surprising, but the best ways to counter it are:
Challenge inaccurate information
Provide accurate information yourself
One of the key aspects of this is to provide evidence. If you make an objective statement without any sort of disclaimer about your uncertainty, that should mean you've got good reason to believe you're correct. That doesn't necessarily mean you need to provide evidence, but as soon as there's disagreement, evidence is king. [...]
Another source of evidence is documentation and specifications. Be aware that they're not always accurate, but I generally believe documentation unless I have a specific reason to doubt it.
In practice, I generally say things like "I believe" or "If I remember correctly" if I'm going by things I remember about games I played previously and/or am not sitting in front of. I find that even when I feel sure that something happened a particular way in a game, I'm more often than not wrong when I trust my memory, and I feel like there should be a disclaimer when I'm feeling sure I know, versus when I have multiple and/or authoritative references.
If my source is non-authoritative, I'll say something to the effect of "(game) community consensus is" or "forum threads indicate".
If I'm citing a fairly authoritative source (ie, a game wiki or FAQ) I try to judge the reliability of the source and note this when posting the reference.
In general I try to focus on providing as much information as possible when I answer, and sourcing that information and it's trustworthiness as thoroughly as possible.
Sometimes, especially with ITG and other "no canonical sources exist" questions, the only way they get answered is with an educated, "best information we have indicates" answer. Sometimes I get it wrong, and I've learned to live with the fact that, I, as a person, am capable of being incorrect.
Once I invent agentBot2000, though...