From what I understand, any interactions she makes will seem like something normal to outsiders. So if she moves a glass in the air, it might just look like someone else is moving it, or more likely they won't notice that the glass is floating at all. However, wouldn't it be fairly easy to prove her existence by having someone hold a # of fingers behind their back, and then Miyagi just tells him the number? Just by having her tell the guy about things that he shouldn't be able to see from his line of sight would be pretty easy.