You can easily recall basic, widely known knowledge of your world and slightly advanced knowledge related to your specialty (up to the DM's discretion what knowledge is considered widely known and advanced). When you attempt to learn or recall something related to your specialty, you almost always know the best place or person to find it from. Usually, this information comes from a library, scriptorium, university, or a sage or other learned person or creature. Your DM might rule that the knowledge you seek is secreted away in an almost inaccessible place, or that it simply cannot be found. Unearthing the deepest secrets of the multiverse can require an adventure of even a whole campaign.