There is truth and truth in the faeries eyes. They can never tell a lie about things they have directly experienced or know themselves, but they can and do interpret events however they like. Harry feels guilty about the events of Proven Guilty, Mab knows this and is offering an interpretation that could be valid based on outside observable events. We as readers know that Murphy was fighting faeries to resolve a problem that affected the citizens of Chicago, but it was just as valid to say that she deserted her post in the face of the enemy to run off with a friend. We know that Harry didn't really entrap Molly since we know his thoughts as the events went down, but that doesn't mean that he isn't carrying some unresolved guilt about not picking it up earlier. From an outside perspective, the great detective and wizard Harry Dresden could never miss something so obvious right under his nose. Whenever what the Fey are saying is opinion or conjecture, they can basically say whatever they want.
Wizards can absolutely tell other talents. If they touch a practitioner with skin on skin contact, they get kind of a static spark or electric current. The strength of this is dependent on the strength of the talent, so full bore mages give off enough and minor talents like the paranetters give off a very little. Butcher explores that in multiple books, particularly Summer Knight, Side Jobs and Cold Days. Dresden also picks up the Skinwalker in Turn Coat just through the emanations of it's power, which were so strong he felt them through a veil.
As for Charity, she willed her ability to do magic away and thus gave no spark to Harry. She basically used the off switch to remove it from herself but she couldn't remove the ability from being passed on to her offspring. That off switch is evidently available to anybody with magical power, although once flipped it can never be recovered.
This quote from Side Jobs sums the whole thing up fairly concisely
“Murdering someone with magic? It leaves an odor, and there isn’t a body spray on earth that can hide it completely so soon after a kill. If Harry got close enough to sense a whiff of black magic on her, there wouldn’t be any way she could pretend to be a damsel in distress.”
“He’d still be able to tell she was a practitioner.”
“Only if he actually touched her,” Bob said. “And even then, if she’s significantly different from a normal human, mentally, it’ll alter the sense of her aura. Besides, sensing a little tingle of magical potential in a client is a whole lot different from realizing that she’s spattered in supernatural gore.”