We do not know for sure. Here are the quotes:

from chapter 6

As his hand touched the back door's handle, he heard a last whisper from behind him.

"Hermione Granger."

"What?" Harry said, his hand still on the door.

"Look for a first-year girl named Hermione Granger on the train to Hogwarts."

"Who is she?"

There was no answer, and when Harry turned around, Professor McGonagall was gone.

from chapter 8

"Were you told to wait for Harry Potter on the train to Hogwarts, or something like that?"

"No," Hermione said. "Who told you about me? "

"Professor McGonagall"

It doesn't look really like her style though.