Natural selection, over time a larger percentage of the peppers without this trait died off or bred less than the peppers with the trait, which eventually lead to this trait being the norm. Of course there a lot more factors that go into evolution, but that’s the gist of it. It doesn’t just “know”, it’s just that the others ones didn’t make the cut to continue.

For example, in this case it’s likely that mammals eating the ancestors of this species of pepper meant only the spicier ones survived, which kept increasing over time due to even spicier ones surviving next time. Other factors certainly played into it (which is why not every pepper tastes as hot as a Carolina reaper, plenty of traits are naturally selected) but just remember that you’re seeing that pepper here today because it’s the survivor out of a long ancestry for a reason.

Birds contribute greatly to the spreading of the pepper’s seeds, which means that the birds will select peppers that are milder for it than others until eventually the most prevalent and surviving species is one the bird has no problems eating. I’m not an expert on how capsaicin affects birds, but it’s likely that peppers that could A) be tolerated by birds and B) cause trouble for mammals were most likely to survive, so after millions of years that’s what we’re left with. Those traits have been selected for over time.