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ignoring the request for a reference), or even get a reference to a past contest, where our ongoing contest policy is no longer in effect.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.