It's usually pretty easy to spot most interview questions on the forums. Not always, but it doesn't take any particular perception skills to know that, "What are the differences between temp tables and table variables?" is frequently an interview question. Can be a valid forum question, in which case, answering it the next day works for a valid question, and defeats the interview-hack (the interview is probably long-since over). Foolproof? Nope. But it's probably as good as we'll get on this situation.

That works. If you think it's a test/homework/interview question, ask. I do that pretty frequently. Again, delaying the answer by a bit is usually enough to make it still valuable to someone who's on the forums to learn, and to defeat the purpose of a cheat.

The interviewer isn't usually trying to defraud anyone. They're doing their best effort to fix a business-problem by paying for expertise. Not always the case, but it is most of the time. It would be dishonest of a business to hire someone and then not pay them, but it's not dishonest of them to hire someone to fill a gap in their knowledge assets. It's difficult, and has risks, but it's honest. Trying to interview for a job you can't actually do is inherently dishonest. It's trying to take money from someone without providing any value to them. Definition of fraud/theft/scam.

We can't catch every attempt. Some will be more clever than we can deal with in this medium. No way around that. Something like 80% of all murders in the US go unsolved every year, and a lot more effort and expertise goes into those than will ever go into trying to catch out interview/exam cheats, and for darn good reason (not even vaguely the same order of magnitude). So I'm sure we miss quite a few. But that's not a valid reason to ignore the ones we could prevent, or at least call-out.

Generally, I start out by trusting everyone. Assume that even seemingly dishonest/unscrupulous looking activities are probably not because of actual intent. Life is more pleasant when you assume you can trust people till proven otherwise. But I will ask, "This looks like an interview question. Is it?", if it does look that way. And quite a few people will admit that it is, and then I'll work with them so they can learn for future interviews. I'll refer them to source material (usually MSDN or TechNet) that they can study for their next go. Most often, even the people who are trying to "cheat", will straighten out when confronted on it. Only about 2.5-3% of humanity is willfully criminal, but some of the rest need some guidance on matters of ethics, etc.