This thread on rpg.net asks why secret magical settings in games tend to be dumb.
My problem is, it's never made any sense to me on an absolutely fundamental. The idea in most of these settings is that humans not only don't believe in the supernatural, they'll even go so far as to doubt their own personal experiences and rationalize them away, or at the very least refuse to talk about them for fear of being labeled insane. That sort of thing would make sense in the real world, where everyone (or close enough) knows magic doesn't really exist. But the reason everyone stopped believing in magic in the real world is because it actually doesn't exist. In a fictional world where magic not only exists but there are monsters that kill people, I see absolutely no reason humanity would stop believing in magic. It'd be suicidally stupid on a species-wide level.
Is it going to depend on what your magic effects are?
When magic is showy then it's going to be hard to hide it. Fireballs, teleporters, flying stuff, people walking through walls, turning into werewolves, eating people - all of that stuff is going to be reported, captured, measured, verified.
When Mage or Werewolf came out in the early 90s, not everyone in the developed world had a cameraphone. It'd be a different world now.
The pitch for Qabal was that magic would not be able to be measured. The importance in magic was the INTENT and not the IMPLEMENTATION. There may be a TELLTALE but for the most part there's only INTUITION to help you deduct a connection.
Look at The Omen for a great example.
Damien Thorne, the son of the devil, had several telltales. The black dog was one of them but another was found in the photographs.
While developing the pictures of the day, Jennings notices the priest has a dark object like a javelin over his head in the pictures he appears in, but the anomaly doesn't appear anywhere else on the film.
Jennings is also at the event, taking pictures. The pictures again show the dark anomaly above the priest.
As the priest leaves, a sudden rainstorm comes up and the priest is impaled in a freak accident while trying to get into a nearby church.
Jennings said he is now involved because he found an anomaly on a picture of himself in which he has no neck.
Jennings goes after the knives, saying he will do it, and is decapitated in a freak accident.
Damian wouldn't have engineered the process of the falling church spire or the panes of glass. He'd have registered his intent that they die. And magic would take care of the rest.