Even having a conversation about foci is too far, you're getting into mechanics. No one ever gets into mechanics about their abilities. We don't need to, and trying is kind of a flag we're trying to get a story ahead of the truth.
Also worth noting:
Prophets, and the gift of prophecy? Are a thing in this universe.
If checking was essentially a free action, every noble of Winter would routinely be checked.
The Gatekeeper routinely screens Winter's soldiers at the Outer Gates for Outsiders trying to infiltrate in the bodies of the injured. Its his job. And he explicitly says that he learned to detect Nemesis from the Fates, who in this universe apparently are the Mothers(see Dresden summon Mother Winter).
I nodded, thinking. "Okay," I said. "First, how do you know if the adversary has . . . infested someone?"
"Experience," he said. "Decades of it. The Sight can help, but . . ." Rashid hesitated. I recognized it instantly, the hiccup in one's thoughts when one stumbled over a truly hideous memory gained with the Sight, like I'd had with—
Ugh.
—the naagloshii.
"I don't recommend making a regular practice of it," he continued. "It's an art, not a skill, and it takes time. Time, or a bit of questionable attention from the Fates and a ridiculously enormous tool." He tapped a finger against his false eye.
I blinked, even though he didn't, and looked up at the massive gates stretching overhead. "Hell's bells. The gates . . . they're . . . some kind of spiritual CAT scanner?"
"Among many other things," he said. "But it's one of their functions, yes. Mostly it means that the adversary cannot use such tactics effectively here. As long as the Gatekeeper is vigilant, it rarely tries." The horns sounded again, and the muscles in his jaw tensed. "Next question."
So its a capability we KNOW they have.
If you think that Mab would not trade a favor to screen her own fucking daughter and heir, my dude, you dont understand her.
And literally screening Winter's soldiers is the Gatekeeper's job.
Like BronzeTongue said, you do not NOT respond to a bomb threat.
Or in the US, a report of a mass shooting.
No 911 operator is going to ask you to prove it when you call in that someone in your building is making a bomb.
No, Winter does not have an easy way to check even the most critical personnel and high nobles. Why? Because it doesn't. If it had, it would screen at least Lady, Queen and Mother routinely. Like, there would be a regular meeting with both Mab and Titania being present, where they would check each other. If they don't do it, they can't do it,
You're explicitly wrong about this. Its been established.
In canon, it took Mab time, but she figured out her daughter was Nfested independently; she knew by the time of Small Favor, which was why she was so furious she couldn't talk to people with her own voice without damaging them, and she didn't have any way to do anything about it until she got Dresden as Winter Knight.
There's a reason her FIRST job for Dresden as Winter Knight was to kill her daughter.
Four years after we know she was aware it out.
Why?
Winter Royalty cannot arbitrarily kill each other. There are Rules.
The Winter Knight, on the other hand, can execute Winter Royalty if necessary. Its part of his job description.