But what did the passages in Egyptian mean? Did they contain exactly the same message as the Greek passage? The last sentence of the Greek text said, "This decree shall be inscribed on a stela [slab] of hard stone in sacred [hieroglyphic] and native [demotic] and Greek characters," so it seemed clear that the inscription was the same in all three languages. That way, the priests' statement could be read by Egyptians who understood Greek, as well as by those who knew only one or both of the Egyptian languages. But the scholars were still far from being able to decipher either the hieroglyphs or the demotic writing. —The Riddle of the Rosetta Stone,
James Cross Giblin

How did the scholars know that the inscription was the same in all three languages on the Rosetta Stone?

The last sentence explained that the inscription was the same.
They deciphered the hieroglyphs on the stone, which told them the inscription was the same.
The British scholars explained that the inscription was the same.
They deciphered the demotic writing, which told them the inscription was the same.