When Jane receives the letter from Caroline (announcing the Netherfield party's departure to London), Elizabeth is very aware of Jane's reaction. The narrator says:

Elizabeth saw her sister's countenance change as she read it, and saw her dwelling intently on some particular passages. Jane recollected herself soon, and putting the letter away, tried to join with her usual cheerfulness in the general conversation; but Elizabeth felt an anxiety on the subject, which drew off her attention even from Wickham

Elizabeth and Jane were very close, and Elizabeth was able to see that Jane was upset even before she knew what the letter said (or even, for sure, who wrote it). This little detail almost makes me wish I had a sister. :-)