How do you know if the person didn't believe the prediction is likely to happen? (benefit of the doubt?)

Depends on the situation, I think. Did the person otherwise behave assuming their prediction is true? Have they told anyone else that they don't really believe it? Etc.

I wasn't claiming that that Musk (or anyone in particular) was lying in my my comment. I was just pointing out that you can, in fact, lie through making speculative statements.

Ah fair enough, in the context of the thread I made that incorrect assumption that was what you were inferring.

"This year you will be able to drive coast to coast with no interventions".

"Tesla indicated that Elon is extrapolating on the rates of improvement when speaking about L5 capabilities. Tesla will not say if the rate of improvement would make it to L5 by end of calendar year."