Stars would show them their position, but it would not tell them about islands.
Birds would tell them that an island was accessible, since they don’t go far from land. It’s also possible that the ocean was lower as well, allowing less distances to travel land masses.
I also think it was a bit of a crapshoot and not everybody survived their attempt at finding new islands. Anybody on islands are descendants of the lucky ones.
Aside from the illusion we perceive from the Mercator Projection. The tip of South America is actually not at all very far from where they had already landed. In Easter Island, New Zealand, and the Islands East of New Zealand.
https://www.popsci.com/uploads/2023/04/21/PolynesianVoyageMap.png
https://i.redd.it/znbl8z0tqcla1.jpg
Also, clouds form over islands and can be spotted from quite long distances.
