Lore-as-magic-sense is something the GM will generally prompt you to roll.
Trapping: Mystic Perception
Exposure to arcane lore leads to a heightened sense of the supernatural. In certain circumstances, you can substitute Lore for Alertness to pick up on supernatural details about a scene. These details should be extremely vague, limited to statements such as "You're getting a bad feeling about this" or "Something about this place is just... wrong." Used this way, Lore is more an indicator that there's something weird happening rather than a lens through which to see the weird.
- Dresden Files: Your Story p.135, "Lore"
So, how does Alertness work?
Trapping: Passive Awareness
You will rarely ask to roll Alertness - if you are actively looking for something, Investigation is usually more appropriate. Alertness is reactive perception. That is, Alertness is more appropriate for things that you do not expect or are not looking for - such as the spiked pit trap in the hallway you're casually walking through. It's a skill that, typically, the GM calls for you to roll.
- Dresden Files: Your Story p.121, "Alertness"
Sticking the two together produces this conclusion: if there's magic about that the GM wants you to be aware of, they'll ask you to make a Lore roll to find it. But at a baseline you'll need to do some additional research or investigation to pick up on exactly what's going on, though a large margin of success on the Lore roll might obviate some of that.