They knew - Either from their former acquaintance with the island, or from the information of the inhabitants.
Was called Melita - Now called “Malta.” It was celebrated formerly for producing large quantities of honey, and is supposed to have been called Melita from the Greek word signifying honey. It is about 20 miles in length from east to west, and 12 miles in width from north to south, and about 60 miles in circumference. It is about 60 miles from the coast of Sicily. The island is an immense rock of white soft freestone, with a covering of earth about one foot in depth, which has been brought from the island of Sicily. There was also another island formerly called “Melita,” now called “Meleda,” in the Adriatic Sea, near the coast of Illyricum, and some have supposed that Paul was shipwrecked on that island. But tradition has uniformly said that it was on the island now called “Malta.” Besides, the other “Melita” would have been far out of the usual track in going to Italy; and it is further evident that Malta was the place, because from the place of his shipwreck he went directly to Syracuse, Rhegium, and Puteoli, thus sailing in a direct course to Rome. In sailing from the other Melita to Rhegium, Syracuse would be far out of the direct course.