Kabuto knows about the Shinigami because he outright told Obito that summoning the four Hokages is out of question, since they are all sealed inside the death gods stomach. Probably Oro and Kabuto did some research after Oro had lost his arms to the technique.
Oh, right. But it doesn't mean he'll know when it's coming. Hiruzen only knew because of the seals. Orochimaru didn't even know, and I doubt he does even now. If I recall, he was never able to see the handseals Hiruzen used since Orochimaru was also in the genjutsu.
Minato's "main" fighting style has a notable lack of handsign usage, so Minato suddenly pausing mid-battle and preforming a bunch of handsigns is gonna be suspicious even if Kabuto doesn't know the exact techniques. Between that and Kabuto knowing Minato would have it available, making an educated guess wouldn't be much and Kabuto loses nothing by being cautious. At worst, Kabuto wastes a trick against some minor technique.
But what about the seals? The handseals look pretty long, and there's no way he could learn how to do that without breaking into Konoha and finding the scroll, if that even exists.
And so far, what we've seen is lack of handseals in Minato's fighting style, but we've seen him mainly use taijutsu and Hiraishin. What if Minato has other jutsu that require handseals?
Which is where the educated guessing comes in. Kabuto doesn't have to know for sure that it's coming, he merely has to wait for Minato to stand still and start making handsigns to act. Even if he's wrong about what Minato's doing, it's not as if he'll lose something by dodging. He could even prepare his own opening as he splits/dodge, since Minato would finally be out in the open.
True, he just has to watch out for Jiraiya protecting Minato. Though, wouldn't it be funny if Minato failed to seal Kabuto and got a snake's soul instead. That'd be the ultimate fail. Jiraiya would lose this alone, since Kabuto can counter Jiraiya perfectly.
Yeah, Kabuto's best bet would be to start with his White Rage technique, which would keep both of them occupied and near defenseless.
Probably with the sound genjutsu first as that's quicker than Jiraiya or Minato summoning the frogs and them tryin to use the sound genjutsu, especially since Minato isn't fast enough for Kabuto out of Hiraishin and Kabuto can tank most attacks.
Kabuto and orochimaru just knew how dangerous that jutsu is. They knows nothing about that jutsu's info. And I think it's freaking retarded if kabuto just keep on guessing if minato would create a hand signs for a jutsu or for his DG. And not to mention, he doesn't have the luxury to do that. Jiraiya would transform him to a frog.
The point is that he knows it exist and thus should be ready for it. I.e. Kabuto knew that the brothers would attempt to catch him in a genjutsu, thus he was prepared to counter their efforts specifically.
Considering Orochimaru was subjected to it firsthand, they would in fact know quite a bit about it afterward. Considering that Minato's main fighting style lacks handsigns, it would be obvious that he's up to something if he stops to make handsigns and thus Kabuto should take caution. It would be ridiculous for Kabuto not to react to Minato acting differently. Nor does he lack the luxury of doing so. Aside from the fact that doing that would also protect from any assault from Jiraiya, the majority of his defensive tactics would also enable him to create an opening for his own assault.
Its so easy to mask handseals its not even relevant if he knows it or not. Just pop some smoke or block his vision somehow. No big deal. JMan was able to block the vision of Pein... That guy also had linked vision.
Wouldn't Minato have to think of that first? Not like that's a regular tactic of his nor would be very secretive act. Not to mention it would work both ways, keeping Kabuto hidden from Minato and making an attack that involves targeting a person even harder to use. And Kabuto would still be aware that Minato is standing still there, so what would prevent him from launching his own attack and disrupting Minato's plan. Both the bone forest and inorganic manipulation would enable Kabuto to strike without having to move. Or by employing his genjutsu/White Rage to stop them both in their tracks.