Minato is a great sensor. To determine how great a sensor is, you can notice his sensor field. Ao's sensory unit for example needed large equipments just to sense Kumo's battlefield. Great sensors like Minato and Tobirama however are capable to sense even countries away(from Konoha to Kumo) and all that without equipments
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Minato can sense chakra build up since he has shown the capacity to notify the change in chakra when a Bijuu is extracted. This is particularly interesting especially when you know that a Bijuu's power can't even be sensed inside a Jinchuriki unless his chakra comes out from one or another reason, so there is no possible ways to notice if one is even a Jinchuriki or a normal person, let alone know whether a Bijuu gets extracted or not, yet Minato did just that. On the picture above you can also notice that even before that, during his revival, he was capable not only to notify Naruto's chakra inside the nine tails countries away, but notice
he completely controlled the Kyubi and they were fighting together
As a counter-example for example, Naruto who managed to sense Minato's chakra battlefields away as well [
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], only noticed Minato's control over the nine tails when he actually used it infront of him [
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]. That speaks volume concerning Minato's sensing abilities
Sensors are capable to sense chakra build inside the body(like you can see above with Minato as well)
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It helps to prevent even occular attacks, which means before Itachi even thinks about puting Minato in a genjutsu, he would know
Also, even in case you're caught inside a genjutsu(outside tsukuyomi of course) a sensor can identify it directly by noticing the disturbance in the flow. It works for every genjutsu since even Shisui's KA was detected by Ao
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