How did Indy know not to look into the Ark?

There was a deleted scene that explained it:

A plot element involving the Ark of the Covenant was cut from the film and is only hinted at during the finale when the Ark is opened.

Basically, there were 2 rules about the Ark not mentioned in the final cut of the film:

• If you touch the Ark, you die


• If you look at the Ark when it is opened, you die

This is first explained in additional dialogue for the scene when Indy and Sallah visit Imam. Before translating the writings on the headpiece that give the height of the Staff of Ra, Imam warns Indy not to touch the Ark or look at it when it is opened.

^[Source]

The next scene involving this Ark subplot is when Sallah and Indy remove the Ark from the Well of the Souls. When Sallah first sees it he reaches out to touch it. Indy stops him before he does and reminds him of Imam's warning. Then they insert long poles through each side of the Ark to lift it out of its crypt.

^[Source]

Notice that nobody ever touches the Ark throughout the rest of the film until the finale.

And from the Bible (King James Version):

1 Samuel 6:19:

And he smote the men of Bethshemesh, because they had looked into the ark of the Lord, even he smote of the people fifty thousand and threescore and ten men...

2 Samuel 6:6-7

And when they came to Nachon's threshingfloor, Uzzah put forth his hand to the ark of God, and took hold of it; for the oxen shook it. And the anger of the Lord was kindled against Uzzah; and God smote him there for his error; and there he died by the ark of God.