When Harry and Ron are hiding in the teachers' wardrobe, this is what they hear:
"The Heir of Slytherin," said Professor McGonagall, who was very
white, "left another message. Right underneath the first one.'Her
skeleton will lie in the chamber forever.'"
Professor Flitwick burst into tears.
"Who is it?" said Madam Hooch, who had sunk, weak-kneed, into a chair.
"Ginny Weasley," said Professor McGonagall.
It's not clear what exactly was the message. Professor McGonagall reports that it said 'Her skeleton will lie in the Chamber forever'. Whether the real full message specified which particular person or not is still not clear. However, one of two things can be inferred:
The message does not say which student it was, but the teachers do a role call in all the Houses and find out who is missing.
Since Ginny was also bait to lure Harry towards the Chamber by the memory Tom Riddle, the full message itself said that it was Ginny (so that the message would reach Harry quickly and clearly) (although, as already mentioned, this is just speculation).