Failing mask

When someone bests the observation ob to see through the mask spell , they see the wizards true form. Do they also see that magic is in play, for example a distorted or shimmering face?

That’s answer is completely up to you and your specific campaign. There are no mechanical guidelines.

That said, seeing through the Mask does not break the spell by any means. So yeah, it could be that the viewer sees that mask as transparent, shimmering, distorted or whatever.

I’d go with the idiom of magic in your campaign first. If it’s not specifically addressed, and I’ve certainly never had this question even remotely central to magic, I’d go with the intent of the observer. The observer wants more information, and thus can perceive both the spell and what lies under it.

Because the guard who shouts at everyone that the dark-eyed, wild-haired wizard is among them isn’t going to be very helpful when he can’t describe the tow-headed youth in peasant garb everyone else sees.