What is the evidence of this? I remember Shirou being excellent at sniffing out active Magecraft, but that IS like a bloodline. Rin was all but useless at that, and while she was decent to good at sensing inert Prana, I don't remember such a thing being superior to a natural Chakra sensor of Naruto.
Nope, there WAS a metal poster in the story a bit earlier than that, but it might have been a different poster. Taiga hit Shirou with it, but she forgot it was metallic and knocked him out.
@DarkKing98 Magecraft DOES do some things much better than Chakra/ninjutsu.
@Deathbybunnies has a point.