One of the most well known of the saints is St. Peter (for whom the Vatican’s Cathedral is named). According to the Bible, St. Peter led his fellow 12 Apostles after Jesus was crucified, and as the leader of the early Church, he is considered by the Church to be its first pope. He is typically depicted with a set of keys (which has been incorporated into Papal coat of arms, as a nod to its first pope) because Jesus is supposed to have told him that he would give St. Peter the keys to the Kingdom of Heaven.
Another saint that was a popular subject among Renaissance artists is St. John the Baptist. Generally, St. John is recognizable via his animal hair tunic.
Saint Jerome’s major identifier is his tamed lion.
St. Paul is identifiable via his long, pointed beard and (usually) bald head. He is also generally holding a sword, an allusion to his martyrdom (he was beheaded by Romans), and a book, a reference to his epistles.