Mortal mages come in a wide variety of strengths and ability, with people strong enough to be proper wizards and therefore perform a variety of different types of magic being very rare.
However, all mortals with the magical oomph to be wizards share two core abilities; initiating soul gazes and looking at things with something called wizard's sight.
For wizards the eyes are literally windows to the soul; the first time they look into the eyes of another soul bearing being (typically other mortals) the initiate a special sort of vision. The details vary wildly from wizard to wizard, but they essentially gain insight into your character, personality, life history, and other things that compose the person they're looking at. Not a full list, and it's usually pretty abstract, but enough to know who they truly are.
As they experience this vision, the other party gets the same view into the wizard's soul in turn.
Harry used this to determine if Molly could be saved from being a warlock in canon, so it's likely that he'll at least want to try this once we get back to Chicago. He can see things like Denarians and mind control when looking at someone like this, so if he does he's probably going to get at least something on the nature of what happened here. Lying about the details he can verify would be bad.
Wizard's sight is a much more powerful and dangerous ability. A proper practitioner can at any time open their magical senses to view the world as it truly is. They can see the flows of magic and emotion in the world around them, pierce illusions of all sorts, and determine incredible information on whatever they look at with it. To my knowledge it's all but impossible to fool or deflect this ability.
The problem with it is twofold. First that when they say "see everything as it truly is" they mean it. Wizard's sight has no safety or filter; you see whatever is before you in all its horrific glory. A lot of things are flat out cognitohazards for mortals, and even the ones that aren't can be so awful as to be traumatic just to glimpse.
The second problem is that they can never forget even the slightest detail about what they see. They'll be able to see that memory just as clearly as the moment they laid eyes on it for the rest of their lives. Just mentioning it to them can call what they saw to the front of their mind's eye again.
As far as Harry knows the only remedy to seeing something awful with the sight is to meditate on it intensely until looking on whatever horror you saw doesn't bother you so much that you're incapable of acting anymore.
Wizards who abuse this tend to go crazy, so they try to avoid using it too much. That said, they still apply it at need.
We should probably try to convince Harry to not gaze directly at an infernal exalted if we can do so without suspicion. The work of the primordials has got to be pretty high on the trauma scale. Or at least convince him to look at our sword instead of directly at Molly.
