I'm not sure what you mean by "how did they know" ? Most culinary inventions aren't a matter of "knowing" so much as some chef combining existing ingredients in a new fashion. The first dishes based around lacto-fermenting fish with rice came to Japan from China sometime in or before the 8th century, and many variations of this dish - some of which might be called sushi, or proto-sushi, and some of which might not be considered related to sushi - would be created over the next millenium, such as Haya-Nare. Likewise, Nori has existed as a food in Japan for since at least the 9th century.

At some point, some chef would have been the first to combine them together, though probably not in any sort of "wrap" method since early nori was eaten in small pieces or semi-liquid. But I think phrasing it as "knew to wrap rice in nori to make sushi" implies there is some obvious advantage or conclusion to these ingredients being combined in the modern form of sushi, which is not necessarily the case - modern sushi is just one possible way of combining these ingredients that has been and is popular, but there are other dishes and many other possible dishes that also use both nori and rice.

My Japanese is terrible and I can't seem to find a decent translation, but supposedly Minoru Katada's book The Rise and Fall of Asakusa-nori says that nori being processed into flexible "sheets" first began in the 18th century and using it to wrap sushi only became common after that.