I argue that anything later than the scope of the question is considered a spoiler.
For instance, if the question is about the Chuunin Exams arc in Naruto, an answer from an earlier arc (such as the Zabuza arc) shouldn't be marked as spoilers.
If the answer is from a later arc (Rescuing Sasuke, for example), those details should be marked as spoilers.
Aside from that, make sure your title isn't spoilerish, and if it is, generalize it to make it good for reading:
How did Character X break out of Edo Tensei?
Is problematic if Character X broke out of Edo Tensei very recently in the manga, which may spoil it for anime viewers.
How can Edo Tensei be broken?
Is better. It only implies that Edo Tensei can be broken, which isn't very spoilerific. Inside of the questions, also in spoiler tags, should be the detailed question, with some introduction message:
Warning! The following contains spoiler for Naruto anime viewers!
Spoiler content here
That's my perspective on how we should be treating spoilers.