What to do when you find a question that should not be on SFSE but you do know the answer?
- Flag it - check
- Answer it - maybe?
- Downvote?
What to do when you find a question that should not be on SFSE but you do know the answer?
It depends...I say just follow your instincts and you're probably right.
Yes, flag it regardless of your other decisions.
As for answering it, if you feel inclined to do so, do it! It's all about helping people. If you think you can give them the help they need and get them an answer faster, more power to you.
If it's a really deep issue and complicated answer, it's probably better off to avoid it, especially since you expect it to be closed or migrated. Typically I would consider offering an answer to quick JS/CSS questions even if they belong on Stack Overflow, for example
Your decision to downvote should be entirely independent of whether the post belongs on a different forum. If it is poorly researched or shows little effort, it deserves to be voted down. If it is well researched but just in the wrong forum, perhaps not.
I'd agree with all of Adrian's points, typically I find that low quality questions/answers tend to get asked by new Users who don't quite get SFSE's etiquette, if that's the right term. How many people can say the honestly read the How to Ask when the first joined? I definately didn't. I would argue new people tend to have a problem they just want help with. Even if the question has been asked a dozen times before, if it wasn't on Page 1 of Google's results it'll get asked again.
So with that in mind, on your third point I'd argue for leniency at least in the beginning and use downvoting sparingly.
Secondly, depending on the question I would say answer it. If you can help someone, help them. If the question is "How do I display a list of Accounts using Apex in Visualforce", I'd say just answer it. Yes the answer is everywhere in and outside of SFSE, but for the sake of one line of code it isn't worth reprimanding someone over and potentially losing a user. We're a community that wants to encourage participation.
On your first point, as Adrian said, flag it regardless. It at least helps give more people visibility of a question that's been asked by someone new who doesn't know any better yet, or a question that's been asked by someone who's earn some notoriety.