When it becomes a fact?

[robvalue]

Personally, I speak informally most of the time.

If I state something is true, or not true, it's shorthand for "All the evidence supports this position, beyond reasonable doubt". It would get tiresome to talk that way all the time. If I'm having a scientific discussion, then I'll be more precise with my language. Someone can always ask exactly what I mean, anyway, and a level of uncertainty is always implied by any reasonable person.

I assume most people treat things this way. Whether I challenge someone depends on the context and what they are saying. If it's important enough, I'll challenge it, if I think they are mistaken or they display too much confidence.