Considering it was 500 years ago when very few navigational tools existed sailing in that was little more than hope and prayer
They had a few tricks that they used though. They knew about the trade winds, for example, and they knew that even if they couldn’t spot land, if they could spot clouds, or spot birds, then there would be land in that direction. Still, it must have taken a lot of courage to be part of those first generations of sailors who took the risk of sailing the open ocean, out of sight of land, and never knowing if you could get back.