How to determine if a question is from a current Contest.

I had never really heard of online contest math question before coming to this site. However, now that I am answering question, I would like to avoid these questions (and flag them). How do you go about matching these questions to current contest? Is it basically just typing terms in from the question in a search engine and trying to find matches, or are there specific sites you look at, or something else?

Thanks.

• I don't check every question. My heuristic: if the question looks like a homework problem, but is too hard for homework, then I start searching if it's a contest problem. There are also certain dead giveaways (integral constants of the form $20XY$ appearing in the problem, where $X\in \{0,1\}$, for instance.) – user7530 Jun 6 '15 at 20:34
• I don't think you really need to - I generally assume that contest problems will be spotted by people who were already familiar with the contest, and that they will hopefully leave a comment and flag. But if a question seems up to the standards of the site and there's nothing which makes me suspicious, I don't go looking. (In the rare case where I do get suspicious, I just copy and paste part of the question into Google) – Milo Brandt Jun 6 '15 at 20:57
• The contest questions that I often see are programming questions, those that look for an algorithm. – Joel Reyes Noche Jun 7 '15 at 1:27
• Occasionally there is also some post on meta mentioning an ongoing contest, like this one. Also sometimes there is post on meta pointing out that some particular problem from a contest appeared here, like this one. – Martin Sleziak Jun 8 '15 at 5:13
• If a question mentions test data or asks for the number to be expressed modulo $10^9+7$ or the like, it’s probably from a programming contest. (Of course it may be from one that has already closed, in which case it may be a perfectly reasonable and even interesting question.) – Brian M. Scott Jun 10 '15 at 1:48

In general for seemingly hard problems without a reference, looking like C&P you can (in addition to voting to close once you're at 3k rep) check google if a contest comes up. As user7530 also noted, questions including a current data (year, for example) often come from contests (or past contests).
You should ask the OP directly if the question comes from a contest and ask him to reference it. From the OPs reaction you can sometimes tell if he is trying to cheat to consolidate your opinion ("This is not a contest problem!", ignoring the request for a reference), or even get a reference to a past contest, where our ongoing contest policy is no longer in effect.

If he doesn't react and you are really in doubt, try asking in chat if someone has seen this or flagging for moderator attention with a message that explains that you are sure it's a contest problem but can't find the contest to it. This will draw attention of other users who potentially know the contest and can confirm your doubt, or give you another source.