If it's a good question and it has a good answer, even if it were really off-topic, I would probably let it stay because it's not hurting anybody and it's probably helping people.
If it's awful or off-topic, has no answers and is cluttering up the home page, then I'd be more tempted to take a shotgun to it and clear it away.
You also have to remember that our guidelines have evolved as we've encountered different types of question. There are some older questions that came before the rules and we generally accept that it's not going to be feasible to fix them. We tend not to delete these but we may need to close them if they're pulling in a lot of undesirable activity.
I certainly wouldn't suggest (or even condone) going hunting for old questions, now outside the rules. There's plenty of new stuff that needs editing, answering and dealing with that takes vast priority.
And it's certainly not something to flag for Mod attention unless it's criminally bad. We have enough fun with the new content issues tyvm.
Oh and with regards to the new dupe question, dupe-close. As always.
an old question would be considered off-topic if posted now, but was never closed as off topic, and
a new question is posted, that is a duplicate of it.
Close the new question as a duplicate of the old question, if it's really a duplicate. There's no reason for people not to benefit from content that already exists on our site.
That's the closest I can come to giving the "single answer" that Oli quite correctly says doesn't exist. Really, more information is needed, so if you are ever unsure about a particular question, you might consider discussing it with others in chat and/or posting a more narrowly tailored meta question about it.