It is really hard to tell with products though. Sometimes the core idea is right, but you haven't yet refined it to match what your audience wants (ie, sometimes you're building for one person that you're not marketing to). Also sometimes it takes a long time to win over your target audience, even if your product is "perfect". In short, sometimes you're doing things correctly, but just gave up too early.

Of course, it is also possible to stay with a bad idea forever. Personally I don't think it's possible to know which scenario you're in (good idea but gave up too early, vs bad idea) until you either succeed or die.