According to The Riddle of the Rosetta Stone, what detail helped scholars determine that the second Egyptian inscription was a simpler form of Egyptian writing that had been created by “the people”? The scholars had seen examples of it before written on rolls of papyrus, a writing material used by the Egyptians.
Since the scholars had already studied Egyptian languages, they recognized both the hieroglyphs and the demotic version.
The Greek inscription, which had already been deciphered, explained how to translate each of the Egyptian passages.
Since only ancient Egyptians understood hieroglyphs, the scholars knew the second inscription had to be a simpler version of the first.