How did Zipporah come to know that God was going to kill Moses?
Exodus 4:24–25 (NASB)
Now it came about at the lodging place on the way that the LORD met him and sought to put him to death. 25 Then Zipporah took a flint and cut off her son's foreskin and threw it at Moses' feet, and she said, "You are indeed a bridegroom of blood to me."
Moses surely would have told Zipporah about God's command to circumcise. While we have to speculate about the details, the Pulpit Commentary suggests a plausible sequence of events.
The suggestion is that it was the eighth day after the birth of their second son; that the first son had already been circumcised; and that Zipporah talked Moses into not circumcising the second.
The narrative of verses 24-26 is obscure from its brevity; but the most probable explanation of the circumstances is, that Zipporah had been delivered of her second son, Eliezer, some few days before she set out on the journey to Egypt. Childbirth, it must be remembered, in the East does not incapacitate a person from exertion for more than a day or two. On the journey, the eighth day from the birth of the child arrived, and his circumcision ought to have taken place; but Zipporah had a repugnance to the rite, and deferred it, Moses weakly consenting to the illegality.
The commentary goes on to suggest Moses may have become seriously ill, and recognized it was a judgment for his weakness. He no doubt would have said something to Zipporah, who then might have needed to do the circumcision because of Moses' weakness, but complained about it. According to the commentary, the complaint likely referred to both circumcisions.
The commentary acknowledges that the language suggests a miraculous appearance, but that it could refer to an illness as well.
It could be that Moses had already told her about the God of Israel and His command and consequences regarding circumcision. Moses had 2 sons, he probably circumcised the first one and Zipporah didn't like it. Therefore she refused to let Moses circumcised the second one. And when God came upon them and confronted them about this issue. She immediately knew what was it all about and what to do next.
[This is just assumption, Bible did not reveal these details]
We would have to assume that the Lord appeared to Moses in a visible and threatening way. Though the text isn't explicit, this was apparently one of the many Theophanies in the life of Moses, and often with these experiences, those around are involved somehow, and to some degree or other.
When Yahweh chooses to appear in the physical realm--either personally or through the agency of angels--there will be no mistake as to His intentions. Everyone who witnesses it will understand His purpose in doing so, I would expect.
