ones you have enough evidence against to justify it as a legitimate threat. Obviously, this wouldn\'t stop everything since people slip by all the time, but like soully said, these things are unavoidable. Theres no way to stop every single threat and know which ones are real and which are fake, or even catch the signs of a silent killer like you pointed out.
the best i think we can do is try our best to catch things before it happens, and finding ways to get as few people killed as possible when something happens.