Testing obviousness with the benefit of hindsight isn't always fair. :-) One test for non-obviousness is if the problem has been known for a long time, but no

Testing obviousness with the benefit of hindsight isn't always fair. :-) One test for non-obviousness is if the problem has been known for a long time, but no one has solved it; or (even better), the standard approaches that are taught for it lead away from your solution.