See this question. I know it's off-topic because it lacks an auido/visual artefact from the game but I do know the answer.
I answered it in this case, but I'm wondering what's the right thing to do.
You can comment or answer but if you answer make sure that your answer is on-topic.
Keep in mind that Answerable != On-Topic. There are a lot of questions that are answerable by some or even most of the people on Arqade but that doesn't make them on-topic.
Helping other users or visitors of this site should be one of the main goals of Arqade and just because something is off-topic it doesn't mean that you're not allowed to help.
Answering an off-topic question isn't bad practice in my opinion and if the question is closed leaving a comment to help the asker is also a nice thing to do. You can even invite them to a chatroom if you want to help them with a topic that would be off-topic on the main site.
A lot of bad or off-topic questions can still attract some excellent answers so don't hesitate to write an answer to a bad or off-topic post (but you answer should obviously also be good and on-topic).
I generally tend to answer in the comments when I see an off-topic or bad post I think I know the answer to. Mostly because i'm not 100% sure if I can help (which is especially true with ITG-Questions).