She finds out the next morning, when Mr. Bennet calls Elizabeth into his study to congratulate her on her connection to Mr. Darcy.
Apparently Mr. Collins has written to (1) congratulate Mr. Bennet on Jane's engagement and (2) warn him that Lady Catherine isn't too happy about Elizabeth's engagement to Mr. Darcy.
Mr. Bennet thinks this is totally hilarious, since they obviously hate each other.
(But we have to wonder: where did Collins get this idea? From Charlotte?)
Elizabeth laughs too, even though she really wants to cry.