There is a store on a street corner that looks a little out of place. It's unobtrusively attention-grabbing, though other people are still just walking by without looking at it. Through the window, it looks like an antique store; the faded wooden sign over the door and window says "Wake's Curiosities".
(It is quite a lot like the store its proprietor walked into to get awakened as a witch. It's not quite as single-topic; she does sell actual curios, some of which are subtly magic. People who don't have potential but do have suspended disbelief in the supernatural come in regularly. Gwen likes visitors more than she dislikes hiding her feathers and keeping people from walking off with magic that will weaken consensus reality too much.)
If someone is walking down the street and has witch potential, they find the sign inexplicably easy to read, and the lights in the window inexplicably attention-grabbing. You don't have to walk in... but if you're not in a hurry, why not?