how does meg know that charles wallace's mind and heart have been taken over

When the food comes, they agree to eat, and just as the man said, Charles Wallace only tastes sand. Charles Wallace then makes a deal with the man, that if he will show him his father and allow him to leave when he wants, he will give into the man’s spell. Though Meg and Calvin protest, Charles Wallace convinces them to let him try. He gives himself over to the man’s stare and within moments, he looks and talks different than the old Charles Wallace. Meg screams that “That isn’t Charles!”