Well, there are some people who are experts in their own world like the Harry Potter crowd, but your best bet for magic expertise in this world are the Winchesters, especially Ruby who was a witch when she was human.
As a whole, the Seal still mystifies most of us. It'll bring people and send them back at random and sometimes it'll do something really weird like take sweets out of existence. No one's figured out a way to predict what it does just yet.