Famously, when Columbus (and several other European explorers) landed in the New World, they thought they'd reached Asia. Hence, for example, our use of the word "Indians" for the local peoples. Even those who didn't believe Columbus's claim of being able to sail west to Asia must have questioned their beliefs for a moment: after all, he'd landed somewhere, and where could it be but Asia?
Eventually, though, it became obvious that it wasn't Asia, and the European powers got down to business exploring and claiming and colonizing and such.
But what exactly tipped them off? What were the things they pointed to and said, "I don't think this is Asia, because..." Were there Europeans who were well-travelled or well-educated enough to accurately know what China looked like, and to know that this wasn't it? Did they hear reports from the East along the lines of "We don't know who you're talking to, but it isn't us"? How did they know that the stories of magical lands to the East weren't simply exaggerated, or that they hadn't landed in a poor, unsettled equivalent of the Philippines, off the Asian coast?
How did the Europeans deduce that they'd discovered a new continent, rather than arriving in Asia?